@NotDesigned Did you install steam from the Ubuntu Package/Software Manager? If not, I would recommend trying that to see if it fixes things or not. After that see if the Curl package is installed.

I've tried it. But still not work

@Anupalabdhi

Ok, I'll go through a step by step checklist:

1. First ensure that you uninstall steam completely (both the one you downloaded as deb and the one you installed from package manager)
2. Second, install steam package from command line (sudo apt install steam).
3. Run steam from command line again.
4. Post log.

@Anupalabdhi I'm having this exact problem in Ubuntu 18.04.
I followed your checklist and installed steam-launcher using apt install steam.
Then I ran steam from the command line, which gave this output:

steam                                                          
Running Steam on ubuntu 18.04 64-bit
STEAM_RUNTIME is enabled automatically
Pins up-to-date!
Can't find 'steam-runtime-check-requirements', continuing anyway
/home/jenny/.local/share/Steam/ubuntu12_32/steam
ERROR: ld.so: object 'libgtk3-nocsd.so.0' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS64): ignored.
[2021-02-19 11:13:40] Startup - updater built Oct 28 2020 23:34:22
ILocalize::AddFile() failed to load file "public/steambootstrapper_english.txt".
Installing breakpad exception handler for appid(steam)/version(0)
[2021-02-19 11:13:40] Loading cached metrics from disk (/home/jenny/.local/share/Steam/package/steam_client_metrics.bin)
[2021-02-19 11:13:40] Failed to load cached hosts file (File 'update_hosts_cached.vdf' not found), using defaults
[2021-02-19 11:13:40] Using the following download hosts for Public, Realm steamglobal
[2021-02-19 11:13:40] 1. http://client-download.steampowered.com, /client/, Realm 'steamglobal', weight was 100, source = 'baked in'
[2021-02-19 11:13:40] 2. http://media.steampowered.com, /client/, Realm 'steamglobal', weight was 1, source = 'baked in'
Installing breakpad exception handler for appid(steam)/version(1.0)
[2021-02-19 11:13:40] Verifying installation...
[2021-02-19 11:13:40] Unable to read and verify install manifest /home/jenny/.local/share/Steam/package/steam_client_ubuntu12.installed
[2021-02-19 11:13:40] Verification complete
[2021-02-19 11:13:40] Downloading Update...
[2021-02-19 11:13:40] Checking for available update...
[2021-02-19 11:13:40] Downloading manifest: client-download.steampowered.com/client/steam_client_ubuntu12
Installing breakpad exception handler for appid(steam)/version(1.0)
Installing breakpad exception handler for appid(steam)/version(1.0)
Installing breakpad exception handler for appid(steam)/version(1.0)
KeyValues Error: LoadFromBuffer: missing {   (current key: '<!DOCTYPE') in file manifest [offset: 14]

/data/src/tier1/KeyValues.cpp (2999) : Assertion Failed: Error while parsing text KeyValues for resource manifest
/data/src/tier1/KeyValues.cpp (2999) : Assertion Failed: Error while parsing text KeyValues for resource manifest
Installing breakpad exception handler for appid(steam)/version(1.0)
ERROR: ld.so: object 'libgtk3-nocsd.so.0' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS64): ignored.
crash_20210219111341_7.dmp[31187]: Uploading dump (out-of-process)
/tmp/dumps/crash_20210219111341_7.dmp
[2021-02-19 11:13:41] failed to load manifest from buffer.
[2021-02-19 11:13:41] Failed to load manifest
[2021-02-19 11:13:41] Error: Download failed: http error 200
[2021-02-19 11:13:41] Saving metrics to disk (/home/jenny/.local/share/Steam/package/steam_client_metrics.bin)
[2021-02-19 11:13:41] Error: Steam needs to be online to update.	 Please confirm your network connection and try again.
crash_20210219111341_7.dmp[31187]: Finished uploading minidump (out-of-process): success = no
crash_20210219111341_7.dmp[31187]: error: libcurl.so: cannot open shared object file: No such file or directory
crash_20210219111341_7.dmp[31187]: file ''/tmp/dumps/crash_20210219111341_7.dmp'', upload no: ''libcurl.so: cannot open shared object file: No such file or directory''
[2021-02-19 11:13:44] Shutdown
/data/src/tier0/threadtools.cpp (3658) : Assertion Failed: Illegal termination of worker thread 'Thread(0x0x56d48300/0x0xf4f99b'
/data/src/tier0/threadtools.cpp (3658) : Assertion Failed: Illegal termination of worker thread 'Thread(0x0x56d48300/0x0xf4f99b'

If I go to client-download.steampowered.com/client/steam_client_ubuntu12 in my browser, I get redirected to a load of spam.

I have exactly the same issue.

ILocalize::AddFile() failed to load file "public/steambootstrapper_english.txt".
[2021-02-20 23:28:15] Verifying installation...
[2021-02-20 23:28:15] Unable to read and verify install manifest /home/rick/.steam/debian-installation/package/steam_client_ubuntu12.installed
[2021-02-20 23:28:15] Verification complete
[2021-02-20 23:28:15] Downloading Update...
[2021-02-20 23:28:15] Checking for available update...
[2021-02-20 23:28:15] Downloading manifest: client-download.steampowered.com/client/steam_client_ubuntu12
[2021-02-20 23:28:15] Download failed: http error 0 (client-download.steampowered.com/client/steam_client_ubuntu12)
[2021-02-20 23:28:15] Downloading manifest: media.steampowered.com/client/steam_client_ubuntu12
[2021-02-20 23:28:15] Download failed: http error 0 (media.steampowered.com/client/steam_client_ubuntu12)
[2021-02-20 23:28:15] failed to load manifest from buffer.
[2021-02-20 23:28:15] Failed to load manifest
[2021-02-20 23:28:15] Error: Download failed: http error 0
[2021-02-20 23:28:15] Error: Steam needs to be online to update.	 Please confirm your network connection and try again.
[2021-02-20 23:28:19] Shutdown
threadtools.cpp (3225) : Assertion Failed: Illegal termination of worker thread 'Thread(0x0x56bd4190/0x0xe9fbab'
threadtools.cpp (3225) : Assertion Failed: Illegal termination of worker thread 'Thread(0x0x56bd4190/0x0xe9fbab'

I've tried installing through the package manager, and through the deb.

This is perhaps also the same as #7667 . I also get these Download failed: http error 0 for urls that should work.