Hi,
Please update your Acymailing to the latest version and check if the license issue persits.
In case the issue persists go to Acymailing->Configuration->License. Copy your license key. Unlink your license, click save, link your license again and enable the automatic sending again. If this does not solve your issue, please open a support ticket (https://www.acymailing.com/support/) and include login credentials for your bacend, so we can have a look at it.