I'm Building a Custom Activity Using C# But It Showing No Compatiple version for Windows Project?

[ppr (Peter Preuss)]

Also have a look here:

Migrating Legacy Activities to Windows

Hi everyone, We conduct regular content reviews at UiPath Marketplace to uphold user experience and trust by ensuring the listings meet the necessary standards and provide value to users. As you might already be aware, legacy .NET Framework 4.6.1 is no longer supported by Microsoft, hence, we encourage all of you to finalize migrating your legacy activities to Windows and keep the content updated with latest product releases. This way you can ensure that the automations that you have developed…

And here:

Activities - Creating Activities With Code