You may have inactivated the 'Completed' Status in the Task Fields or may be the 'Closed' flag is not checked for the 'Completed' Status in the Task Fields, 'Status'.
You can follow the instructions below to check and take the necessary steps to fix this issue.
Navigate to Salesforce 'Setup' -> Quick Find 'Task Fields' -> Scroll down to the 'Status' Field and click on the hyperlink to open the Status Field Values.
Check if you have the 'Complete' Status value in the Inactivated values list. If so click on 'Activate' link
Make sure that 'Closed' flag is checked for 'Completed'. If not click on Edit to update and save it.