If the Quick Start Attack fails with the message:
Failed to attack the URL, please check that the URL is valid
then the first thing to do is check your URL in a browser.
If it works ok then open the ZAP Manual Request Editor, replace the default URL with the one you are trying and send the request.
The response may well explain the problem.
Browsers will try variations of a URL that ZAP will not attempt, and we have also seen sites blocking ZAP scans based on the request signature.
In the latter case scanning the sites using a proxied request rather than using the Quick Start Attack button was successful.
Are sometimes impacted by items covered in the SSL FAQ resulting in failures when using Quick Scan.