For PIPE simulation, certain parameters are required, and might need to be manually set. These required parameters are provided in the example design. When you generate an example design from the Vivado IP catalog, all required parameters are set, and no additional action is required. However, custom designs will require that you add the following parameters to your design test bench file.
defparam board.AXI_PCIE_EP.xdma_0_i.inst.pcie4_ip_i.inst.PL_SIM_FAST_LINK_TRAINING=2'h3;
localparam EXT_PIPE_SIM = "TRUE";
defparam board.AXI_PCIE_EP.xdma_0_i.inst.pcie4_ip_i.inst.EXT_PIPE_SIM = EXT_PIPE_SIM;
defparam board.RP.pcie_4_0_rport.pcie_4_0_int_inst.EXT_PIPE_SIM = "TRUE";
defparam board.RP.EXT_PIPE_SIM = "TRUE";