Please check if the machine.config file on your machine (in c:\windows\microsoft.net\Framework64\config) contains:
<add name="dotConnect for MySQL" invariant="Devart.Data.MySql" description="Devart dotConnect for MySQL" type="Devart.Data.MySql.MySqlProviderFactory, Devart.Data.MySql, Version=8.13.1366.0, Culture=neutral, PublicKeyToken=09af7300eec23701" />
where, the version number is likely different, but that's the gist of it
If you're running the designer in 32bit mode, don't, use the normal exe. Perhaps you started the 32bit instance by accident as that one uses a different machine.config file (as .NET 32bit keeps these separate) and it's likely the provider isn't installed there.
I also assume this is with v5.6, tho you didn't mention that.
As your previous message in another thread was about EF Core 3, just a heads up: we support EF Core on SQL Server, PostgreSQL and Firebird, not MySql.