To switch over to the free FreeAgent tariff:
Email the FreeAgent support team (support@freeagent.com), explaining that your free trial ended before you were able to move over to the free Mettle tariff
FreeAgent will then send you a link to switch your account to the 'FreeAgent for free' tariff
Once that is complete you can then reconnect using the same process you attempted before.
Please note that you are only able to have one account on the Mettle 'FreeAgent for free' tariff.