To share some personal experience.

I got same error about active DEF tank, and since I live in Toronto, took the tank myself to Xemodex in Mississauga. Picked up next day, took it to a mechanic to install.

After picking the car now I have a error after seeing another CEL warning.
'no reducing agent build up pressure'. It seems tank is letting DEF out via the vent on top. So there will be another drop of tank and taking for warranty service. Such a hassle.