Description:
This happens when your top-level design is in VHDL but one of the
lower-level modules (for example, PLL generated using IP Express) is in
Verilog.
Solution:
Take note that Lattice OEM Compiled libraries have different nomenclatures for VHDL and Verilog. For example, "sc" is the SC/M library name for VHDL, and "ovi_sc" is the SC/M library name for Verilog. When you use vsim/asim command, make sure that you use appropriate libraries.