The lender won’t see the accept renewal button (and it won’t show in their renewal requested queue) until the borrower requests another renewal. The borrower needs to add a Desired due date and click Renew Item on their side before the request for renewal will be send and appear for the lender.
If the lender still does not see the Accept Renewal button, clear cache and cookies in the browser.
If problems persist, contact OCLC Support with your library symbol and the ILL Request ID so they can help with additional troubleshooting.
There is more on requesting renewals for Interlibrary loan items.