Before proceeding, it is essential to address a common misconception: Microsoft has never released a “.NET Framework 5.1”. The classic .NET Framework versioning ended with .NET Framework 4.8.1 . Version numbers 5.0 and higher refer to the modern, cross-platform .NET (Core) .
Install .NET 8.0 Runtime (x64) for modern apps, and enable .NET Framework 4.8.1 via Windows Features for legacy software. There is no official .NET Framework 5.1. download net framework 5.1 windows 10 64 bit