We have a F450 and the exhaust fluid light is on and the computer shows"Speed limited to 50 MPH in 99 miles" and when we try to add fluid it bubbles over. What should we do?

This vehicle is a military vehicle so we need to get it back to the unit and we dont have much experience with vehicles needing exhaust fluid so it is new to us.  So the dash is showing that the vehicle will be limited to 50 mph in 99  miles.  I know that if the fluid gets very low that the vehicle will ultimately idle only.  We have some Adblue fluid to put in but we think there is still a little of the fluid from the factory in the tank.  When the guys tried to add fluid it just kept bubbling over.  Please help!