Receiving the message when approving is infact not an error, but a warning. As it is stated in the message all claims must first be viewed. It is meant as a safety precaution to not approve unwanted or incorrect expenses.
In Approval expand the claim using the < icon
Then expand the detail line, again using the <
Open the claim via View
It may be necessary to click View again depending on your configuration
Use the Green Approve button to approve the claim and it will process