Hi,
I guess you already ran the migration process some time ago?
In order to run the migration process once more you can execute this query in your database editor:
UPDATE
#acym_configurationSET
value= '0' WHERE
#acym_configuration.
name= 'migration'
Don't forget to replace # with your database prefix.
Then access the AcyMailing 6 Security page and the migration button should be visible again.