An error on top of header

Something in your project is sending an HTTP header before it is supposed to happen. This results in this message. I have the following tips for you:

Deactivate all plugins as a test. Does the message disappear then? Then it was due to one of the deactivated plugins. You can find out which one was at fault by reactivating them individually.

As soon as you have found the cause, you must decide what to do with it. If it is a plugin that you absolutely want to use, contact the plugin’s support.

The fact that the message can be seen at all indicates that the output of PHP warnings is activated in your hosting. This should only be activated for debugging purposes but not permanently. This can happen if you have WordPress itself in debug mode (see: https://wordpress.org/documentation/article/debugging-in-wordpress/) or due to a setting on your hosting. If it is the latter, please contact your host’s support team for clarification.