I have a connon printer MG6620 (old printer) and it says I need to contact third party vendor for it to work with my new imac Is there any other way to connect the printer?

According to this Apple document, About AirPrint - Apple Support, your model is AirPrint compatible and doesn't need any drivers. Get out its manual and see how to set it up wirelessly in your LAN.

Please do the following in this order:

·Uninstall all Canon software, put in the Trash and empty the Trash.

·Please reset the printing system per Reset the Mac printing system to solve a problem

·Next, follow the instructions in Adding a printer