I am trying to do a checkout. When the error:
error: Unable to open .git/HEAD.lock for writing
happens, SourceTree, thinks it succeeded but it actually failed.
Here's the full output in the "Switching branch" dialog with "Show Full Output" checked:

git -c diff.mnemonicprefix=false -c core.quotepath=false reset -q --hard HEAD –

git -c diff.mnemonicprefix=false -c core.quotepath=false checkout master
error: Unable to open .git/HEAD.lock for writing
Switched to branch 'master'

git -c diff.mnemonicprefix=false -c core.quotepath=false submodule update --init --recursive
Completed successfully.

I've also attached a print screen.