UiPath Custom Activity with target platform x64 bit

I have created a custom activity by using ML.NET that supports only x64 target platform. But unfortunately, on installing custom activity that targets x64 platform is not showing up in Activities panel in the UiPath Studio.

Platform Any CPU and x32 is working perfectly fine and on installing they simply showed up under Activities panel in UiPath studio version 2018.3.2.

Question:
Is there any limitation of target platforms to make a custom activity?

Any help will be appreciated. Thanks in advance.

we are also looking to use the ML.NET in AI Center, is this any why to Add ML.NET in costome AI model