To add a devices your role should be admin or super admin then only user can able to add a device.
Step 1:
From the left-side menu, click on "Devices" > Go to the "Device Details" page > On the top left of the page, click the "Add Device (+)" button.
Step 2 :
The "Add New Device" form will appear, fill in the required details.
Step 3 :
After entering all the necessary information, click "Add New Device."