After setting up the "IP RTL Library Path", please click the "Add Ref" button to select the reference RTL file (libraries) in the IP RTL Library Path. Note that this file is not packaged into the IP and will serve as a reference file only.
For more information, you can check page 28 of the FPGA-UG-02187-1-0-Lattice-IP-Packager-2023-1-User-Guide document.
You can download it here: https://www.latticesemi.com/view_document?document_id=53856