If either your project directory or the Quartus® II software installation directory contains spaces, you may recieve the following error message:
Error (12006): Node instance "gen_duplex_xcvr.u_phy" instantiates undefined entity "altera_xcvr_custom"
This is due to the SDI MegaCore® function failing to generate all of the required files needed for compilation.
To workaround this issue, please ensure that both the Quartus II software installation directory and the project directory do not contain any spaces. Then re-generated the SDI MegaCore function in order to have all of the required SDI MegaCore files in you project directory.