I've installed LittleMsSam's More Buyable Venues mod, and my sim can purchase a Lounge, but when I try to open the Lounge to the sim-public, suddenly it tells me I have no items set for sale; when I clicked on the "Business" button, it shows that the lot is a Retail lot, even though it's set as a Lounge.

What do I do to fix this?

I can't set any items for sale, or I would.

How can I own & operate an ACTUAL Lounge?

Having the same problem with Daycare and Medical Clinic

Bet bet is to ask the mod creator and other helpful people over at their Discord Server.

That brilliant mod doesn't function together with the retail system from GTW at all.
don't use ANY options from the retail system while running your business from that mod

don't buy it trough the phone, there's an option on the pc > household > LittleMsSam venues, and don't travel to it using the little airplane icon, use the map instead.
just read the mod description.... it says that clearly and explains everything you need to know..

and if you already jump into LittleMsSam's discord, look into the pinned messages in #support
there's a tutorial that explains how to buy a venue, and how to start your business, I think in the first pin there.