Ensure your custom addon resides in a path that is actually part of your addons_path in the odoo.conf file. If the configuration file has changed, restart the server. 
Also, check the log whether the correct paths are loaded upon starting the server.

Finally, ensure the module is in the correct structure:

/x/y/z/custom_addon/__manifest__.py

/x/y/z/custom_addon/__init__.py

'/x/y/z' must be part of your addons_path in the configuration, custom_addon is the custom addon (folder) and in there at least a  __manifest__.py file must be present. For odoo.sh also a __init__.py must be present.