Fatal error: Class not found in

Plugins are loaded before functions.php. You should include the class in your plugin if possible.

I have had scenarios where a class was part of the theme, but also needed in a plugin where you couldn’t assume the class was included in the theme. In those cases, I simply included the class in both places and wrapped it in a “class exists” check.

Like this:

if(!class_exists('My_Class'))
{
 class My_Class{

    // Class Methods and Properties
 }
}

You could also include the class only once in MU Plugins, which is loaded before the other plugins.