It's probably gonna go into "safe"(LIMP) mode before long@! I'd be tempted to drain it, siphon it or whatever ya gotta dotry to find a place where you can get it to drain from, & then refill with fresh, OR find a tune that will let you shut off the EGR & RUN DISTILLED WATER.
Its not the DEF, I ran distilled water for 4 years in My 2015, error free, That message is related to DEF Injector in most cases, Clean it , clear the code and place the engine in Stationary De-soot mode, If problem persist its the in tank DEF pump.
Thanks TC, I'm still learning % figuring out this engine, my age doesn't help either. I had forgotten about the earlier trucks not having a valid sensor.