Pipelines ASP.NET error he reference assemblies for .NETFramework,Version=v4.8 were not found.

[Dolenc Dolenc]

I try to make Pipelines for my project, I have this yml code:

image: microsoft/dotnet
pipelines:
default:
- step:
caches:
- dotnetcore
script: # Modify the commands below to build your repository.
- export PROJECT_NAME=myprojectName.sln
- export TEST_NAME=BildSolution
- dotnet restore $PROJECT_NAME
- dotnet build $PROJECT_NAME
- dotnet test $TEST_NAME

 But I get this error error:

/usr/share/dotnet/sdk/2.1.811/Microsoft.Common.CurrentVersion.targets(1175,5): error MSB3644: The reference assemblies for .NETFramework,Version=v4.8 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks [/opt/atlassian/pipelines/agent/build/DataAcess/DataAcess.csproj]
/usr/share/dotnet/sdk/2.1.811/Microsoft.Common.CurrentVersion.targets(1175,5): error MSB3644: The reference assemblies for .NETFramework,Version=v4.8 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks [/opt/atlassian/pipelines/agent/build/Common/Common.csproj]
/usr/share/dotnet/sdk/2.1.811/Microsoft.Common.CurrentVersion.targets(1175,5): error MSB3644: The reference assemblies for .NETFramework,Version=v4.8 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks [/opt/atlassian/pipelines/agent/build/WebPortal.Tests/WebPortal.Tests.csproj]
/usr/share/dotnet/sdk/2.1.811/Microsoft.Common.CurrentVersion.targets(1175,5): error MSB3644: The reference assemblies for .NETFramework,Version=v4.8 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks [/opt/atlassian/pipelines/agent/build/DataSet/DataSet.csproj]
/opt/atlassian/pipelines/agent/build/WebPortal/WebPortal.csproj(1223,3): error MSB4019: The imported project "/usr/share/dotnet/sdk/2.1.811/Microsoft/VisualStudio/v16.0/WebApplications/Microsoft.WebApplication.targets" was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.

what can I do?

[Dolenc Dolenc]

Any help?