Hi @Elba ,
SharePoint On-Premise Server Client Object 15.x is not compatible with .NET Core solution.
Currently only SharePoint Online CSOM support for .NET Core (SharePoint On-Premise didn't support) , please check this blog:
For detailed information, please refer this blog:
.NET Standard version of SharePoint Online CSOM APIs
If you are using SharePoint Online, then install the CSOM from Nuget like this:
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