The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)

I know I'm missing something obvious here - I have seen this error before and I just can't remember how I fixed it!

I have my main ProjectX.Web project and my ProjectX.Core project. I'm using Models Builder API and all works fine locally. But when I check it in and try and build up on Azure DevOps I get a load of these errors. So I know I'm just missing something from the checkin.

  ProjectX.Core\Models\File.generated.cs(15,7): Error CS0246: The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)

ProjectX.Core\Models\File.generated.cs(16,7): Error CS0246: The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)

ProjectX.Core\Models\File.generated.cs(17,7): Error CS0246: The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)

ProjectX.Core\Models\File.generated.cs(18,7): Error CS0246: The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)

ProjectX.Core\Models\File.generated.cs(19,7): Error CS0246: The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)

ProjectX.Core\Models\Folder.generated.cs(15,7): Error CS0246: The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)

ProjectX.Core\Models\Folder.generated.cs(16,7): Error CS0246: The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)

ProjectX.Core\Models\Folder.generated.cs(17,7): Error CS0246: The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)

ProjectX.Core\Models\Folder.generated.cs(18,7): Error CS0246: The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)

ProjectX.Core\Models\Folder.generated.cs(19,7): Error CS0246: The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)

ProjectX.Core\Models\Image.generated.cs(15,7): Error CS0246: The type or namespace name 'Umbraco' could not be found (are you missing a using directive or an assembly reference?)