[MusoBob]

Try this it should work on the current time signature and tempo at the cursor:

retval_inputs, bars_set = reaper.GetUserInputs( "Set Bars to Insert", 1, "Insert Bars", "0" )
if retval_inputs then   
  cur_pos = reaper.GetCursorPosition()
  
  timesig_num, timesig_denom, bpm = reaper.TimeMap_GetTimeSigAtTime(0, cur_pos)
  
  time_set = (((bars_set * timesig_num) / bpm) * 60) 

  start_time, end_time = reaper.GetSet_LoopTimeRange2(0, true, false, cur_pos, cur_pos + time_set, true)
  
  reaper.Main_OnCommand(40200, 0)  -- Time selection: Insert empty space at time selection (moving later items)
  
  reaper.Main_OnCommand(40635, 0)  -- Time selection: Remove time selection  
end