This situation is encountered when you have many accounts on Omisell and have connected the current shop. To protect data as well as sync at the connecting account, we do not allow the new account to link again.
However, you can absolutely do it in one of two following ways:
1. Log in to the old account, delete the shop connection.
2. Contact customer support through https://community.omisell.com
To receive support from us, you need to provide information about your account code, shop link you want to connect.