First of all, please check if your IPv4 subnet can be transferrable or not. As example, there’s a two years transfer restriction from the last transfer/allocation date for subnets allocated by RIPE NCC.
Secondly, you need to check if all the annual RIR fees are paid.
There’re much more recommendations, so we prepared a separate article on it. You can read more about our recommendations regarding IPv4 addresses before the IPv4 transfer here