It is clear to me that the OP is asking for help in diagnosing the symptom and fixing the problem. Alas, I can't help any, I don't have any diesel experience; especially with ones squirting in "magic juice" to make them less stinky/smokey.
Before going to far and thinking it is a low pressure situ and start throwing parts at it. Do a reset, make sure def tank is full, clear code and see if code comes back. The reset procedure is in the manual as well as other things you need to read for this segment of antipolution.
Also, may want to do a forced regeneration also.
I have seen codes like this in fords and others where client ran DEF tank to low and truck codes low pressure when all it was, was a low tank.
There are also regular maintenance things that need to be done. Such as particulate filter. If those are so clogged even a forced regen doesn't work, you may need to change that. It can be expensive.
If you have a scanner, you can is some cases activate the def pump and check the pressure against specs, but you need some serious investment if you don't have access to higher end scanners and test tools like I do.
But again, at risk of repeat myself, first thing, fill tank, reset, clear code and drive it at let it do it's own regen's on it own. See what comes back, if any.