If something can be found in both refs/heads/ and refs/remotes/ then this is ambiguous.
You have local branch origin/release_2.6 and remote tracking branch release_2.6 for remote origin. Don’t think you are supposed to have a refs/heads/origin/release_2.6 branch though. Anyway, you can specify full reference name to resolve ambiguity:
git co -b release_2.6 refs/remotes/origin/release_2.6