Game version 5.1
Save from version 5
Solo campaign

Task require repair then refuel the truck but even at 100% for both it doesn't validate the task.

Solution:
The tracking hud show 100% for both but actually i was missing just one point on the suspension then the task got validated.

Mine worked. Not sure why yours didn't. I would refuel and repair it again. Likely missing only one point in something. "Ol' Faithful" and The Great Nomad" will not accept repairs or fuel on mine tho. Wondering if yours worked.