Using the guide, I set up the API on FreePBX 15.

https://wiki.freepbx.org/display/FPG/API+Applications

I was able to get the example shown working with GraphQL and Insomnia.

There is nothing about creating a REST app, so I tried that.

I see I was given similar URL for REST API like the image in the Wiki shows.

Rest URL:http://192.168.1.11/admin/api/api/rest

I plug the URL into the Insomnia application like the GraphQL example shown at the Wiki, and clicked “Send,” but something not expected in the display window.

Page Not Found

The page you are looking for could not be found. Check the address bar to ensure your URL is spelled correctly. If all else fails, you can visit our home page at the link below.

Checking the directory structure, I clearly see no such directory exists.

Here are the only directories found

/var/www/html/admin/libraries/Api/Rest
/var/www/html/admin/modules/fax/Api/Rest
/var/www/html/admin/modules/callforward/Api/Rest
/var/www/html/admin/modules/callwaiting/Api/Rest
/var/www/html/admin/modules/conferences/Api/Rest
/var/www/html/admin/modules/contactmanager/Api/Rest
/var/www/html/admin/modules/queues/Api/Rest
/var/www/html/admin/modules/daynight/Api/Rest
/var/www/html/admin/modules/donotdisturb/Api/Rest
/var/www/html/admin/modules/ucp/Api/Rest
/var/www/html/admin/modules/findmefollow/Api/Rest
/var/www/html/admin/modules/parking/Api/Rest
/var/www/html/admin/modules/presencestate/Api/Rest
/var/www/html/admin/modules/timeconditions/Api/Rest
/var/www/html/admin/modules/userman/Api/Rest
/var/www/html/admin/modules/voicemail/Api/Rest
/var/www/html/admin/modules/core/Api/Rest
/var/www/html/admin/modules/api/Rest

Either the the documentation is out of date, or the installation is not creating the correct folder.