FORTIOS 7.0.5 NAT64 ACTIVATION

Hello 

I WISH TO ACTIVATE NAT64 ON MY NETWORK TO PERMIT USERS WITH AN IPV6 ADDRESS ACCESS SERVERS ON THE INTERNET USING AN IPV4 ADDRESS. I HAD AN ISSUE WHEN I TRY TO REUSE THE IPPOOL USE FOR MY NAT IPV4, HOW DO I SOLVE THIS ISSUE