Entity Framework Core adding to .Net Core Web Api - IRelationalTypeMappingSource problem

Had the same issue, turns out the current version of Pomelo.EntityFrameworkCore.MySql requires a version of Microsoft.EntityFrameworkCore between 3.1.8 and before 5.0.0

Downgrading Microsoft.EntityFrameworkCore to the version before 5.0.0 (3.1.11) solved the problem for me

I decided to change db to Sql Server. I've removed Pomelo.EntityFrameworkCore.MySql and added Microsoft.EntityFrameworkCore.SqlServer and problem's solved.