What do I need to get my iMac to be compatible for Adobe Suite?

Adobe tends to support only the most recent three versions of macOS. At the moment, those would be (Monterey, Ventura, Sonoma).

Sonoma just came out a short time ago, so the requirements page for Photoshop still says Big Sur or later. It might soon read Monterey or later. Likewise for other Adobe apps.

https://helpx.adobe.com/creative-cloud/system-requirements.html

If the software requires a version of macOS later than 10.15.7 Catalina, the only thing you can do is buy a new Mac.

That one has hit the end of its upgradable life. It's 10 years old now. It can't go any further.

I believe that the Affinity V2 Suite (Photo, Designer, Publisher) will run on Catalina.

https://affinity.serif.com/en-us/