Build failed for ASP.NET Core 3.1

I cannot build my ASP.NET Core 3.1 application and after several unsuccessfull attempts I think it’s time to figure out with the community what’s the problem.

Build config:

Here’s my repo in GitHub:

I think the problem is that my travis file is not in the same folder with the solution file, so I put directory path in travis configuration file, but maybe I have some mistakes.

Thanks in advance, @joshua-anderson, @akoeplinger !