3 answers
Hello Molly,
Try to add the "Files" related list on the page layout. If you are in lightning, go to Setup > Object Manager > Task > Page Layouts > Click on the page layout you are using, select the related lists on the gray panel, look for Files and drag it down to the related list, quick save and open a task record to confirm the Upload File button is existing.
Hope this helps.
Regards,
Kervin