Failed to validate the notification URL 'https://ngrokfrwdURL.ngrok.io/api/spwebhook/handlerequest

<?xml version="1.0" encoding="utf-8"?> <m:error xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">
    <m:code>-1, System.InvalidOperationException</m:code>
    <m:message xml:lang="en-US">Failed to validate the notification URL 'https://ngrokURL.ngrok.io/api/spwebhook/handlerequest'.</m:message> </m:error>