If you get any issues like this, your first port of call will be your settings along with the error message received - you'd be surprised how helpful these can be!
Make sure you review your Company Information page -Email
Communication - Practice Email, check through the settings there
Integrations - so if you're using Gmail or M365, this is the place to check. Make sure that the integration is actually authorised for a start
If it hasn't been authorised, go ahead and do so.
If it has, REVOKE access, then re-authorise and try sending the email again.
If you have any further issues, please send us a screenshot and describe the actions taken - contact@support.co.uk