I dealt with this problem and tried many suggestions from various websites: clean def tank and injection from tank. I concluded it was the pump however I ordered a new tank too (likely unnecessary) but since it could not be returned, I went ahead and installed it since it came with the pump already installed.. after installation the count down continued however when it got to zero miles left, the regen started and indicator went off. No more problems.
I had already ordered a new pump so I still have it new in the box if you eliminate all the other causes and are interested.
The DEF injector builds up with crystals of the fluid where it injects into the exhaust and triggers a see dealer for service message. You can remove the injector easily and clean it with hot water and replace it. The injector is held in by a ring clamp with 1 little bolt in it, mine needed a small allen wrench to remove it. The message left after some miles and the check engine light turned off after some start engine cycles with the key. My truck is fine and running well.