When modeling a differential surge tank, the following user notification occurs:
"Exactly two active pipes must be connected to this type of node in its current configuration."
With a surge tank node element using the surge tank type "Differential" there must be two pipes connected to the tank. This is because the external riser (see "Lower (External) Riser" in figure below from the Help) is modeled as part of the differential surge tank node itself (the external riser diameter is entered in the differential surge tank properties). Basically it is assumed that the external riser component that's part of the differential surge tank is connected directly to the water main. This is the intersection of pipes "branch 1" and "branch 2" in the figure below, where one of these "branch" pipes is the water main upstream of the external riser connection and the other is the water main downstream of the external riser connection.
So, if the differential surge tank is only connected to a single pipe, that indicates that you attempted to explicitly model the external riser, which could introduce an error in the calculation since that pipe is already accounted for in the tank.
If you do have two active pipes connected to the tank, one of them may be closed in the initial conditions. Check the computed status of both pipes to confirm. A pipe may close itself automatically if the tank has become full or empty in the initial conditions.
"Simple" surge tank types can be connected to a single pipe (the lateral pipe can be explicitly modeled as a pipe element)