How to fix: In Azure Devops connection string is null despite setting Variable in the build Pipeline

I have problem with build pipeline in Azure Devops, with variables from build pipeline not replacing empty configuration in appsettings.json. Below there is more details.

My current test project is build using asp.net core technology and is connected to SQL server. I also use Entity Framework Core and autofac.

To connect to SQL server I use appsettings.json configuration:

{
  "ConnectionStrings": {
    "AzureDbConnectionString": ""
  }
}

but my credentials are stored in secrets.json

{
  "ConnectionStrings": {
    "AzureDbConnectionString": "Server=tcp:servername-db-srv.database.windows.net,1433;Initial Catalog=dbname-db;Persist Security Info=False;User ID=user;Password=Password;MultipleActiveResultSets=False;Encrypt=True;TrustServerCertificate=False;Connection Timeout=30;"
  }
}

I've configured my build pipeline variable: Name:

ConnectionStrings--AzureDbConnectionString 

Value:

Server=tcp:servername-db-srv.database.windows.net,1433;Initial Catalog=dbname-db;Persist Security Info=False;User ID=user;Password=Password;MultipleActiveResultSets=False;Encrypt=True;TrustServerCertificate=False;Connection Timeout=30;

The problem occurs while running Generate Migration Scripts in the build pipeline.

Autofac.Core.DependencyResolutionException: An exception was thrown while activating λ:Microsoft.EntityFrameworkCore.DbContextOptions[] -> λ:Microsoft.EntityFrameworkCore.DbContextOptions -> λ:Microsoft.EntityFrameworkCore.DbContextOptions`1[[AspNetAutofacAzure02.Data.SchoolContext, AspNetAutofacAzure02, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]. ---> System.ArgumentException: The string argument 'connectionString' cannot be empty.
   at Microsoft.EntityFrameworkCore.Utilities.Check.NotEmpty(String value, String parameterName)

Like I mentioned, it looks like the variable isn't used while generating the script.

Is there anything wrong I do here?