I found the cause. Please ignore the question below.
Hi, I have a very simple design in which I use an iopll and chip_id IPs. The design is compiled fine on Quartus Pro v18. However, Modelsim cannot find the iopll and chip_id libraries when I compile in Modelsim. I think somehow those two libraries are not generated properly from Quartus. I use Intel Modelsim. Can someone please help me with this? I am new to FPGA design. Thanks