Can I use an iMac 2015 as a display for a Mac mini M1 ?

Both your devices are too new. The very restrictive hardware and macOS conditions for what you want to do, Target Disposal Mode, are laid out in this Apple article:

Use your iMac as a display with target display mode - Apple Support

Of the five options listed in the 9to5Mac article, only the external monitor is appropriate. SideCar is about iPads. AirPlay to Mac has proved to be laggy, as are the two third-party solutions.

+1 on just getting an eternal monitor so you are not struggling with the other options.

Apple started the deprecation of Target Display Mode in late 2014, and threw the last shovel of dirt in its face in early 2020.

David Graeme-Baker wrote:

My wife has a 2015 iMac 21.5 inch and I have a Mac mini M1 8Gb is there a simple way to turn the iMac into a display for the Mini ?

That way she gets an up to date computer running all the latest software.

No that will not work.

You need to use an aftermarket external display with the Mac mini and repurpose, sell or recycle the 2015 iMac.

see > Connect a display to Mac mini - Apple Support

No, unfortunately. The iMac isn’t a display (it’s a computer) and can’t be used as one.

there are third-party ways

Use iMac as monitor: 5 Solutions - 9to5Mac

though it would be more enviomental friendly just to get a monitor as it would be using a fraction of runing as whole computer as display for anohter one.