Are you attempting the transfer from this link?
https://www.rscrevolution.com/transfer/index.php

You will have to log into your old RSCLegacy forum account.

If you don't remember your forum password, you should still be able to recover it from that website as long as you used your real email.

Instructions:

1. Login using the username and password of the RSCLegacy forum account for character(s) you wish to transfer.
2. Click "Transfer Characters".
3. If any of your character names are unavailable on RSCRevolution, you will be notified with an error.
4. In that event, use the free name changer in the character manager before trying again.
5. The entire forum account and all its associated characters will be transferred to RSCRevolution.
6. If the forum name is already taken on RSCRevolution, you will receive an error and have to change your forum account name at the RSCL profile settings before trying again.