Never mind, I see it must be a problem with my environment.
How did you solve this?
I did make install and it showed the same error.
Then, I tried, make proton && make install and I still got the same error..
Same issue here and I don't know what to do.
I'm pretty sure I just stopped the VM and killed some process (something sshfs maybe) and tried again.
proton 4.11x1 2019-09
I tried to build proton_4.11 using "make install", which got a long way, but failed with: