On that blue "create merge request" button on the issue page, there is a drop down arrow to let you pick a branch to start from. That text input is super buggy and often says the branch does not exist even if the name is correct.

Some times when trying to merge in a MR, the UI will say the source branch doesn’t exist so the MR can’t be merged. Wait a bit and it usually resolves itself. But it’s super annoying when you are in a rush.