I know that it is not possible to install a library with compatibility Legacy to a project with compatibility Windows. Also, it is not possible to install a library with compatibility Windows to a project with compatibility Legacy.
I understood that your case is not library, but custom activities.
Anyway, it seems to me that it works in the same or similar way.
So, both must be Legacy or both must be Windows.
It seems that your project is Windows and your Custom Activity is Legacy.
In this case, you can keep your Custom Activity in Legacy and create the project again, but this time with Legacy compatibility. The other option is to keep your project in Windows and create your custom activity again, but this time with Windows compatibility.
If I understood it wrong and your project is Legacy and your Custom Activity is Windows, you can:
Keep your Custom Activity in Windows and convert the project to Windows. The other option is to keep your project in Legacy and create your custom activity again, but this time with Legacy compatibility.
NOTE: As I said I know how the combination library + project works. Both have a dropdown to select the compability when creating the library/project.
I’m not sure how it works for custom activities and if it is as clear to choose compatibility as it is for library/project.
Anyway, the same idea applies to the combination custom activity + project
Windows 32 bit (Windows Legacy)
Windows 64 bit (Windows)
Cross Platform (which also works on Linux, Mac, Studio Web).
Cross Platform will work anywhere but has the most limitations as it has to work with things common to all operating systems.
Windows 32 bit and 64 bit are not compatible with each other in any combination.
If you have a dependency (Library or custom activity) in a 32 or 64 bit it can only be used in other 32 or 64 bit versions. This is due to the underlying programming language from Microsoft.
You should make sure both your project and custom activities are in 64 bit since UiPath is deprecating support for 32 bit.