The most common reason for this is that the Beams device is not assigned to ResScan correctly, in the machine data file (MDF).
Check your Machine Data File:
%% Beams
beamDaqDevices = {'PXI1Slot4'}; % <- let's say you have one DAQ device for beams
%% ResScan
beamDaqID = 1; % <- assign that DAQ device to be used with ResScan
Useful ScanImage Links: