Are you running the Steam beta client? Are both accounts in the family sharing beta?
Steam Community has been rather unstable the last few days, would you mind trying again to verify it wasn’t just steam hiccoughing.

I have the same issue on latest beta client, and the latest stable client (tried both), on two separate computers. Wiping the Steam directory (~/.local/share/Steam) doesn't help either, nor does running steam with LC_ALL=C. I'm able to deauthorize, but not authorize. It does not matter which name I type into the device name box either.

Additionally I get the same error when I try to authorize my account as I get when I attempt to authorize my girlfriend's account.

I have the same problem I can authorize any Windows computer (haven't tried Mac) but can not authorize a Linux computer.

sorry, accidentally hit the wrong button!

@johndrinkwater
do both accounts NEED to have family share enabled? or just the account that has the games to share? the same error message is thrown regardless of how you attempt to authorise the account. it doesn't seem that way because it works fine on my other pc (under both linux and wine).

on your other points: both linux PCs are running the beta version of steam; i just tried it moments ago and the same error message is shown on the (tenfoot) linux pc.

it actually works fine on my main desktop PC using both linux and wine. (so its not something regarding my account(s) or necessarily linux itself.) see below (this is under linux on my main desktop pc):

however nothing i do seems to fix it on the other linux PC. any idea what the invalid parameter message means?

The account which is sharing the games requires family sharing permission, you also need to be using the beta client ‐ thats it ‐ for it to work.
As for the invalid parameter bug, I have no idea what the issue is.
One idea, on this problematic machine could you close steam, rename ClientRegistry.blob in the Steam dir to something else, and try this again. If that fails, you can rename it back, no harm should come.

Renaming the ClientRegistry.blob does not change anything.... still the same error message with invalid parameter at least for me.

Is this connected with https://github.com/ValveSoftware/steam-for-linux/issues/2905 ? FS Beta allowed authorization before

I have exactly the same issue. During the early days of the public beta, I was able to authorize two accounts on my Ubuntu laptop. After a later update, I found that I was no longer able to share games between accounts. In the discussions, it was suggested that de/reauthorizing might fix the problem, but once I deauthorized, I've never been able to authorize again, nor on my other linux PCs. I always get this error - likewise when I try to launch a game from an unauthorized account and Steam prompts me to request access.

I see the same error when I try to authorize my laptop running Debian Sid. I was able to authorize my wife's laptop running Windows 8.

@johndrinkwater i tried deleting the blob file and relogging in, and the same errors still occur.

Same issue with 3 differents accounts on Arch linux.
Was able to authorize windows steam under wine.

Exactly the same here, too.
Can't authorize linux machines, only Windows machines.
Halp?

I have the same Issue running Fedora 19 (3.11.10-200.fc19.i686).
Authorization doesn't work with the latest stable or beta Client.

Affects me too. Have got just the same problem on a my child PC under Linux Mint DE.

Same problem here, running Gentoo.

Also having same problem from both my linux machines, running arch.

I just upgraded my machine to Fedora 20 and now it works... so maybe for Fedora people this is a solution.

With the newest updates for Fedora 19 32-Bit (3.12.5-200.fc19.i686) it's now working too

i can confirm this is also fixed for me. (original poster)

This suddenly started working for me, too, two days ago.
No steam or system updates between non-working and working - I think they fixed something server-side?

closing as i think it is working for everyone now