FLStudio's plugin manager is not up to date with the latest versions of the SDK.
This was taken from the page "VST 3 Locations / Format" in the SDK documentation:
[quote]
On the Windows platform a VST 3 Plug ...
FLStudio 20 and the mixer and then I put some Effects to the Mixer. But the problem is, I wanted to delete this Effects from the Mixer slot, but I could not find the "none" in order ...