Hello,
I cannot access your repo (we can only access customer repos if there is an open support ticket from one of the repo admins). However, the relevant error message here is
The current .NET SDK does not support targeting .NET Core 3.1. Either target .NET Core 2.1 or lower, or use a version of the .NET SDK that supports .NET Core 3.1.
What image are you using in your build?
Can you try using the following image and see if the issue is fixed?
image: mcr.microsoft.com/dotnet/core/sdk:3.1
Kind regards,
Theodora