Right click on any merge commit and choose 'Reverse commit...'->OK

The error message is

git -c diff.mnemonicprefix=false -c core.quotepath=false -c credential.helper=sourcetree revert --no-edit 8137de37b91c056d605484aee0c59d93b7a8cdf7
error: Commit 8137de37b91c056d605484aee0c59d93b7a8cdf7 is a merge but no -m option was given.
fatal: revert failed
Completed with errors, see above

if the command also had the option

-m 1

then it would work, however it would of course be better to set m to the correct number of commits that comprise the length of the branch, or get the user to select a valid commit and set m accordingly