Hi Trisha,
As the error suggests the woactivity record is being 'updated by another user.' That user could be a physical user who is also on the record making change to the WO. It could also be something that is updating the record as part of the Workflow that it is getting tripped up on. Finally, it could be getting updated by automated processes like automation scripts, escalations, and cron tasks. You may need to look at a few different things. If you get the error, you might try refreshing your record and trying to run the workflow again.
Without knowing your system and your workflow it is hard to tell.
Thank you,