I assume you have a CLR component in the database project. Sql CLR does not support .net core. also CLR projects do not support .net standard class libraries. I believe you can use the portable library, but this is not supported by core after core 2.0
you can upgrade your database project to 4.8
if you are trying to share code between the core projects and CLR, then create a multi-target project that builds a .netstandard and a net48 library.
the shared code will be limited to C# 7.3
note: SSIS script packages do not support .net core.