If you want to grant your shop manager access to WP SMS, you will require the User Role Editor plugin. To start, download and install the plugin.
After installing follow the path WordPress Dashboard → Users → User Role Editor.
Then you need to change the role to Shop Manager and search SMS and give access according to the picture below:
Now your shop managers can enjoy using WP SMS!
You can also use Send SMS Guide article for more info on how to send SMS in WP SMS.