Hi @anon96390478, welcome to the Asana Community Forum and sorry for the trouble here! This is strange, the “+ Add task” button should appear under the user’s profile picture:
I see they are sorting their My Tasks by Completed tasks, could you please ask them to sort by Incomplete Tasks or all a tasks and confirm if the option appears?
Please also try from a different browser and follow these troubleshooting steps that usually solve this type of issues.
Looking forward to your reply!