Hi Erick,
You can use this casting only in case if method below returns instance of ZCL_CRM_MKTPL_CPG_FUSG_COLL:
lr_fuh_coll ?= ir_mktproject->get_assignment_collection( cl_crm_mktpl_constants=>asg_type_fusg ).
I guess this is not the case in your example that is why you have casting exception. Maybe method get_assignment_collection returns instance of CL_CRM_MKTPL_CPG_FUSG_COLL?.
Regards
Adam