SourceTree tells me file is locked when its not.

[Alex Antonov]

Im merging my commits using "Theirs" files and one of their files is locked.

I rebooted, turned off every process and the problem persists.

I can't work anymore because of that, please help me.

[Seth]

Exactly how this works may vary depending on your diff/merge software, but you should be attempting to save changes from the merge to the final destination only. It has often been the case, in my experience, that the temporary files you are merging from are read-only, and get deleted as soon as the conflict is resolved.