You can use this link to check if your page is a valid AMP or not. If your published landing page is reported as invalid AMP pages, please ensure that you have not added any external scripts or style tag in "Other Scripts" area

or in the additional javascript area inside the builder

Please note that you will not be able to add custom javascript in AMP pages, AMP doesn't support external javascript unless its AMP compliant. This is not a restriction we impose but AMP community does this to ensure that the AMP pages load fast irrespective of the environment. If you are planning to add external javascript, I would recommend standard pages as they are more versatile.