Im getting a this error when I add to the external tool. We wanted to partner up with elsevier and they gave us a url for the lti tool to work however Im getting this message:

Whitelabel Error Page

This application has no explicit mapping for /error, so you are seeing this as a fallback.

Elsevier support can't also find the problem. I tried doing it on moodle versions 3.2 up to 3.5.10 but the error still persists. Hope I can get help with this problem thanks!