Warning: Cannot Modifier Information..Headers already sent

This is often caused by conflicts in plugins. What is happening is that your script is attempting to redirect to a page after content has been written to the header. Once you write content to the header (using echo, print_r, etc) if you attempt to redirect (using a header:location for example) the server is telling you that have already written to the header and cannot redirect.

This article might help you get started troubleshooting this.

https://nickroshon.com/wordpress/how-to-fix-warning-cannot-modify-header-information-headers-already-sent-error-message-in-wordpress