All claims that have the status 'Open' have been created and are displayed in the creator's Self Service page, but they haven't been submitted for approval.
In this case, only the creator of the claim will be able to amend it or submit. You cannot edit someone else's open claim by temp covering as them.
If the creator is unavailable you will need to create a new claim for them and submit it for approval this way. Then, when the employee return to work, make sure to advise them to delete the unsubmitted claim.