you need to disable MacOS's Gatekeeper in terminal using the command

sudo spctl --master-disable

this should allow you to open the Checkra1n.dmg file

if checkra1n does not open and crashes on start,

then right click on the checkra1n icon, and click on show package content,

and navigate to content - MacOS

and click on checkra1n_gui

this should start the application

once you are done with using the application, it is advisable to re-enable gatekeeper by using the command

sudo spctl --master-enable