Error trying to update oh-my-zsh I have already removed yarn, and continue

This looks like a Git warning message - did you changed something down the ".oh-my-zsh" directory ? Especially in the "yarn" subdirectory ?

This Stackoverflow question should help to recover : git: error: you need to resolve your current index first

Your repository of oh-my-zsh have some file modified (it's a git repository). You can do:

1. cd "$ZSH" or cd ~/.oh-my-zsh/ to access the root folder of zsh
2. git add . to add all files (modified/included) as staged
3. git stash -m "your message" to put all files modified/included in stash, it will clean the repository, but you can catch all the files later.
4. omz update or upgrade_oh_my_zsh to upgrade the zsh

After the update you can catch the files that are added in the stash (if you needs) with git stash apply (the stash will be there still) or git stash pop (the stash will be removed)

Note: Apply your stash after the update can produce some local conflicts.

reference of my answer