Because asking for approval could create 1 or could create many approval records, you first need to add a step before you can update your approval record(s).
If there is a chance your flow will create multiple approval records (such as when asking a group to approve) you need to add the "Look up records" action. Add your condition on this action to look up approvals where the approval is for the trigger RITM. (see screenshot below). You could add more conditions to this such as state = x. Whatever suits your business needs.
You then need to add a "for each" flow logic. For each approval record found in the step above, and then add your update record within that.
If you will only ever be asking for 1 approval (such as line manager approval) then use the Action "Look up record" (no "s" on the end). Use the same conditions as above. You don't need a for each loop on this and can go straight to updating the record.
You can use the approval reason field instead
