Site 2 Site VNP with overlapping networks srx300 to srx240

Hi!

I have the followig problem:
Site A: Network 192.168.12.0/22
Site B: Network 192.168.20.0/24 (Networks 192.168.13.0/24, 192.168.14.0/24 and 192.168.15.0/24 are assigned to other services on Site B)

How do I manage to get traffic from 192.168.12.0/22 to 192.168.20.0/24?
I assigned the IP address 172.21.8.1/22 to the st0.1 interface.
I thought to static NAT from 192.168.12.0/22 to 192.168.20.0/24 using 172.21.8.0/22 but srx said that the subnet masks from source to host didn't match (/22 to /24).
I want NAT from 192.168.12.0/22 to this network 192.168.20.0/24 using this transfer network 172.21.8.0/22.
Can please someone tell me how to configure this?

Kind regards
Andy