.NETFramework,version error

I can't run project, the error is

Microsoft.Common.CurrentVersion.targets(1229, 5): [MSB3644] No reference assemblies found for .NETFramework,Version=v3.5. To resolve this issue, install a developer pack (SDK or targeting pack) for this version of the framework, or retarget the application. The developer packs for the .NET Framework can be downloaded from https://aka.ms/msbuild/developerpacks.

But as I know, .net 3.5 is no longer supportedAll components are connected