Hi nonaetets,
We'd recommend first taking a look over the tips here :
Set up and turn on Switch Control on iPad - Apple Support
As this is an external switch, first ensuring it's properly connected to the iPad may address being unable to add it.
"Step 1: Connect an external switch (optional)
If you want to use an adaptive accessory, joystick, game controller, or MFi device for Switch Control, you need to first connect it to iPad by following the manufacturer’s instructions. If the switch connects using Bluetooth, you need to pair it with iPad.
- Turn on the switch.
- Go to Settings
> Bluetooth, turn on Bluetooth, tap the name of the switch, then follow the onscreen instructions.
Step 2: Add switches and assign actions
You can use only one switch, or you can use multiple switches to accommodate different actions. Each switch performs an assigned action, such as selecting an item or moving the cursor to the next item.
- Go to Settings
> Accessibility > Switch Control > Switches. - Tap Add New Switch, then choose External, Screen, Camera, Back Tap, or Sound.
- If you’re trying to add an external switch but it isn’t shown in the list, make sure it’s connected to your iPad as described above in Step 1: Connect an external switch (optional).
- Assign an action to the switch.
- The action for the switch depends on the number of switches you have and the scanning style you choose in Step 3: Choose a scanning style, below. If you don’t know which action to assign, choose Select Item, which is compatible with Auto Scanning (the default scanning style). You can return to this setting later to change the assigned action."
Cheers!