For now, SharePoint Online list validation does not support lookup column. If you need that, You may need to use Power Apps.
You can start from here:
Items list showing ItemTitle and ItemCategory
Create a Lookup Column (ItemOrdered) in Invoice list pointing to Items list
showing ItemTitle and ItemCategory in Invoice list
Invoice List Showing ItemTitle and ItemCategory
In fact you can easily find many examples.
Here are some scripts for your reference:
Tried to figure our where to add these without creating another Answer.
I don't see the 'Active' Status automatically updating on the Member Training List. Though it is updating on the Member List, and the flow says the run 'succeeded'
I got my accounts crossed. Posted a response to you comment as another questions.
Your suggestion worked.
However, the Update item in the flow is forcing (as a "required" field; not my choice), for some reason; I'm not even using in in my work.
This is forcing the Update to update the wrong items because the SP Id no's are not the same for each ...
The way I would handle that is not by having a lookup back to the Members list, but by creating a Flow (Power Automate) that kicks off when items in the Members list are updated, and then finds the item in the Members Training list that corresponds to the "member" that just got updated (via a unique ID, if you have that, or maybe by member name, or ...