MvcJsonOptions
was removed in .NET Core 3.0 and is not available in versions newer than 2.2 See the "Applies to"-Table: https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.mvc.mvcjsonoptions?view=aspnetcore-2.2#applies-to
In case you are using Swashbuckle, this thread suggested updating to the newest Swashbuckle version should help: 'Could not load type 'Microsoft.AspNetCore.Mvc.MvcJsonOptions' from assembly 'Microsoft.AspNetCore.Mvc.Formatters.Json, Version=3.0.0.0