This error is shown because mailwizz does not have a delivery server to pick to send the current campaign.
This can happen for various reasons, such as:
1) You actually have never created a delivery server in the first place.
2) You did create a delivery server, but it is not confirmed.
3) You have a valid delivery server, but it is specifically assigned to a customer other than the one which this campaign belongs to.
4) You have a valid delivery server, system wide (not assigned to a customer), but in it’s settings, it is set to only send transactional emails.
5) You have a valid server for all emails, but the settings from Backend > Settings > Customers > Servers (same for group setting) dictate that the customer must add his own servers in order to send the emails.
6) Everything above doesn’t apply, but still you can’t send campaigns, but you can send test emails for example. Sometimes, the PHP version from the web server can be distinct than the one from command line. For example, apache can run php 5.6 but the command line version can be 5.0. The command line version is the version called by the cron jobs, the tasks that try to actually send your campaigns. To check this, from command line, the command php -v should show you the right php version number for the cli. Keep in mind that some delivery servers, such as amazon ses web api for example, require php >= 5.3.3.