Is it possible to create a account to have FTP Access to a Mount on Platform.sh?

You can use SFTP in place of FTP, which is the Secure version of the File Transfer Protocol (SFTP).

FTP is insecure and that's why FTP access is not allowed: it would cause security risks and make maintaining audit trails impossible.

For more information on this feature, please see the following page in the Platform.sh documentation:

https://docs.platform.sh/development/file-transfer.html