First, my information is that vray 3.7 and Max2021 are not supported by chaosgroup as I don't see official download:
You probably installed Max2020 vray 3.7 version, right ? You can do the same with ForestPack too. Of course we do not support it officially.
When installing FP, select Max 2020 and install to dummy folder. Take ForestVRay37.dll
file and copy it max 2021 folder:
C:\Program Files\Autodesk\3ds Max 2021