Indigo On SUPro2021 but Not on SUPro2019

[Kanastrous]

Hello - am running a Windows 10 PC with both SU2019 and SU2021. I installed Indigo per the instructions and find that while its palette comes right up in SU2020 it doesn't appear in SU2019.

I want to have Indigo available to use in both programs. Is there a way to accomplish this?

NOTE TO MODS - because I am excellent at following directions I posted this question on the general index before reading the READ THIS FIRST notice, please discard the post to the general index in favor of this one here.