github action error "Specify which project or solution file to use because this folder contains more than one project or solution file"

I added a GitHub action to my repository to test my branch and then merged it to the master, but it gave me this error:

MSBUILD : error MSB1011: Specify which project or solution file to use because this folder contains more than one project or solution file.

There are four projects in the solution.

How can I solve this problem?

name: .NET Core

on:
  push:
    branches: [ master ]
  pull_request:
    branches: [ master ]

jobs:
  build:

    runs-on: windows-latest

    steps:
    - uses: actions/checkout@v2
    - name: Setup .NET Core
      uses: actions/setup-dotnet@v1
      with:
        dotnet-version: 3.1.301
    - name: Install dependencies
      run: dotnet restore
    - name: Build
      run: dotnet build --configuration Release --no-restore
    - name: Test
      run: dotnet test --configuration Release --no-restore

This usually happens when you are starting with a single project, and your .sln-file ends up inside a project folder. Easiest way out of this, is to move .sln-file one folder level up.

Additional resource for moving the .sln-file (credit to @Charles): https://stackoverflow.com/a/626646/14072498

Just click on the solution in the Solution Explorer and then click on "Save myProject.sln as..." in the File Menu. This will save your .sln in the folder that you choose without breaking the references.

To fix this without moving .sln-file, follow the list below.

Replace WebApplication.sln and WebApplication/WebApplication.csproj with corresponding names from your app below.

run: dotnet restore ./WebApplication.sln

run: dotnet build WebApplication/WebApplication.csproj --configuration Release --no-restore

For the tests, point to .sln-file

run: dotnet test ./WebApplication.sln --configuration Release --no-restore