How do I connect my HP Laserjet Pro M15a to my Mac mini?

HP Support – HP LaserJet Pro M15a Printer

There are no Mac drivers on the Support site – only drivers for Linux and Windows.

HP's specifications say that the minimum system requirements for Macs are El Capitan, Sierra, or High Sierra, so possibly there were drivers for those versions of macOS that have been removed. (Any such drivers would have taken the form of Intel kernel extensions that would have no chance of running on a M4 Mac mini. Rosetta can't translate Intel code that runs at the operating system level – like Intel kernel extensions.)

There's no indication in the HP specifications or on the Apple Support page About AirPrint that the M15a supports AirPrint. If it did, you would be able to use it with a Mac without the need for separately-installed drivers.

This thread in HP's forums, about problems using this printer with Sequoia, seems to confirm that HP simply hasn't provided up-to-date Mac drivers for this printer. Someone asked "Are HP working on giving support or shall I take my printer and throw it in the trash?", and a Moderator who was a HP employee gave a generic answer saying that 'there may not be an immediate timeline available."

Apple-Silicon-based Macs had been out for nearly four years when that thread appeared on the HP forums – and have been out for nearly five years now. If there have been no new drivers for Apple Silicon Macs and Big Sur (or later) in all of this time, I wouldn't hold my breath waiting for HP to release new drivers now.

How are you actually trying to connect it?

Does plugging in a USB cable from the printer to the Mac not make it show up in System Settings ➜ Printers & Scanners? Drivers should not really be necessary. A USB-A to USB-C adapter will probably be necessary.

click here ➜ Add a printer to your printer list so you can use it on Mac - Apple Support

You probably wouldn't be comfortable with the amount of technical fiddling around needed to make this work, but here is how one person salvaged several old printers that otherwise would be unsupported by macOS.

https://www.reddit.com/r/MacOS/comments/1g4xq7j/comment/ls710mw/

Try the European site for HP for a driver. The European Union mandates that companies must support devices, i.e. printers in this case, longer than in the US.

It appears that the OP started two threads about this same issue.

Here is the other one.

HP LaserJet M15A will not connect to Mac … - Apple Community