How to fix FreePBX 15 The Module Named “manager” is required.

Problem:

When trying to install a FreePBX 15 module like Asterisk REST Interface Users you see the error message

The Module Named "manager" is required.

but you can’t find the manager module in the module list

Solution:

As you can see in module.xml in manager’s GitHub repository (which you can find easily by just googling FreePBX manager), the module is called

Asterisk API

Note that you can just click on the The Module Named "arimanager" is required. message if you disable your AdBlocker.