Cant pair iphone to imac with bluetooth

As the message stated, pairing an iPhone and a Mac is not supported.

An iPhone will not pair via Bluetooth to a computer (Mac or Windows) or an iPad except for personal hotspot and then only if your cellular plan supports it. See the following for more information: Bluetooth: Why can't I pair my iPhone or … - Apple Community

If you are attempting to use Handoff/Continuity or AirDrop, that does involve Bluetooth but the devices are NOT paired; Bluetooth just needs to be on and the devices in range of each other. The following may help with Handoff/Continuity issues: Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

Also see Share files between your Mac and Bluetooth devices - Apple Support

Bluetooth needs to be on for those capabilities but the devices are not paired. The following may have something that will help. See: How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

and

Make and receive calls on your Mac, iPad, or iPod touch - Apple Support