Issue with trying to automate tasks to get marked as completed when moved to a certain stage

Hi,
Yes, the problem is Find Records always find a list of records (sometimes it's a list with 1 or 0 records), but Update record is a single-record operation.
Solution in you case - loop through result of Find Records and use Update record inside the loop. You need to add Repeating group and use list of found records as input list. Then move Update record Inside the loop and adjust its setup. During setup of Update Record, don't forget to switch on 'Current item" when you choose data for record ID to be updated.

I don't think you need "find record" step

What would I do instead? 

So this works but its missing the conditional stage of finding tasks within a stage. When a project gets updated to "Present & Decision", I would like the previous stage tasks to be marked as completed. How would go about for this?

So what you want is when you change the stage of a record you need to update that stage same stage field pf the same record?? Or Am I wrong?

Can you indicate ina screenshot of the data? what is the field that triggers the Automation and what is the field that needs to be updated?

Yeah unfortunately you can only use a repeating group or conditional logic, not both at once.  Not entirely sure what your workflow is like, but with reference to your initial screenshot I'd suggest changing the trigger to be 'When Stage is "Present & Decision"' and then putting in that Find Record, followed by a repeating group with the Update Record action

You don't need conditional part if you change the trigger to "When record matches condition" and set condition to "Stage=Present & Decision"