Allocation Workbench is designed to show only tasks of type Project / Demand.

If Resource plans are created for tasks other than Project / Demand then allocation workbench may fail to load.

Please refer to this KB0750733 (https://support.servicenow.com/kb?id=kb%5Farticle%5Fview&sys%5Fkb%5Fid=31901581db0afb04a39a0b55ca9619de) for more details.