It sounds like an installed plug-in (Retrologue) only came with a temporary license, which has apparently now expired—I'd guess you'll have to check with Steinberg as to your options for obtaining a permanent license, if you want to continue using that plug-in. If you're not interested in using the Retrologue plug-in, you could remove it from the appropriate VST/VST3 plug-in folder (Library->Audio->Plug-Ins->VST/VST3) to get rid of the popup.
