First update packages using

 apt update

Install flamshot using apt or snap

 apt install flameshot

 snap install flameshot

Find the location of flameshot using

 flameshot

then copy path of flameshot and go to settings of Ubuntu
Follow this steps
Settings --> Keyboards Shortcuts and scroll down

Press + button

to command field paste flameshot path and add gui command after the path then press set shortcut button

then you should press print screen button on your keyboard!
It should work if it does not first you should clear default screenshot command search for it and remove it then follow the steps above again!
Happy using!