I'll update the category of this thread and I'll let you know our findings about the error. Looks like they still don't have official version for .NET 8 published in NuGet, only 8.0.0-rc.1.
UPDATE:
The situation is as follows:Npgsql.EntityFrameworkCore.PostgreSQL, Oracle.EntityFrameworkCore and Pomelo.EntityFrameworkCore.MySql don't have builds against the official .NET 8 and we have no other option but wait to publish their builds, until then you can use such data sources only for .NET 7. Previews versions of Radzen Blazor Studio without .NET 8 support can be found here: What's new | Radzen Blazor Studio