Hi MushroomDog71 - I had the same issue trying to get any external microphone to be recognised by my 2019 iMac. This model of Mac only has a headphone jack at the back, but the Apple website said that its headphone jack can be used for an external microphone. After 8 calls to Apple support here in Australia - lovely, helpful people but they couldn't resolve the issue for me - I made-up an adapter (splitter) cable and was successful. If you aren't comfortable making cables and soldering get a friend to help you because you don't want to damage anything.
The 3.5mm headphone socket on modern Macs uses a 4-pole connector - looking at a 4-pole plug that would plug into it, from the top of that plug the conductors (and respective signals) are in descending order Tip (left audio out), (1st) Ring (right audio out), (2nd) Ring (earth/common), and Sleeve (mono microphone in) then the cable comes out this end.
The adapter cable I made used the Tip and 1st Ring for left and right audio channels respectively (they might be reversed but that doesn't matter to me) plus the 2nd ring for earth/common wired to a stereo 3.5mm inline socket for the external stereo headphones (on the stereo 3 pole socket: Tip - left audio, Ring - right audio, Sleeve - earth/common). This side of the adapter does not use the 4-pole's Sleeve conductor.
For the adapter cable's external mono microphone I used the 4-pole connector's Sleeve (mono microphone in) and 2nd Ring (earth/common) wired to a mono 3.5mm inline socket (Tip and Sleeve respectively).
To make this I bought a 4-pole male to 4-pole male 3.5mm extension cable, cut-off and discarded one end, then stripped-back the wires, identified which wires connected to its Tip, Ring, Ring, Sleeve conductors using a multi-meter, then soldered the appropriate wires from the 4-pole cable to the correct bits on the mono, and stereo, inline sockets.
A quicker and easier way to make the adapter would be to buy a 4-pole male to 4 pole male cable, and a 2-pole (mono) 3.5mm extension cable (male one end, female on the other) and cut-off the plug side keeping the inline socket side (female), and do the same with a 3-pole (stereo) 3.5mm extension cable. Then strip the wires back from both the mono and stereo socket cables and solder them to the appropriate 4-pole wires. A bit of heat shrink and/or electrical tape, check your continuities to ensure your wiring is correct, and you are done.
There's a trick to using it - you must plug your external microphone into the appropriate inline socket on your adapter, and the external headphones into the stereo inline socket on your adapter, BEFORE you plug the 4-pole plug into the back of the Mac in the headphone socket. If you do it another way you will probably be able to use the external headphones but the external microphone won't be recognised.
All going well, when you plug-in your adapter cable with the external headphone and microphone plugged in to it, you'll see the System Preferences - Sound - Input change from "Internal Microphone" to "External Microphone" and System Preferences - Sound - Output change from "Internal Speakers" to "Headphones".
Before making my own adapter I bought an adapter "Y" cable from a local electronics shop and while the external headphones worked the external microphone did not.
I'm guessing that most people who need to do this visit their local musicians' store, buy and use an "audio interface" to get external analogue audio into a modern Mac (analogue signal(s) in and digital out through USB.) Easy and you don't get your hands dirty. I saw a Rode adapter cable for sale but didn't want to buy another item in this project, so I can't comment on whether it works or not.
Have fun!
