Specifically, what Apple keyboard are you attempting to use with your iPad?
Unless you are using a battery operated keyboard intended for Mac computers - your iPad will not be connecting via Bluetooth. The Apple Magic Magic Keyboard for iPad, and Smart Folio keyboards that magnetically attach to your iPad, connect using the iPad’s physical Smart Connector.
If you are using a Bluetooth keyboard, the iPad User Guide outlines the process of pairing Bluetooth accessories - such as a compatible keyboard, mouse or trackpad - with your iPad:
Set up and use Bluetooth accessories on iPad - Apple Support
Should you experience difficulties, this support page should help with troubleshooting:
If you can't connect a Bluetooth accessory to your iPhone or iPad - Apple Support
Assuming that you are in fact using a Magic or Smart Folio keyboard for iPad, remove the iPad from the Keyboard - and carefully/gently clean the smart connector contacts on both the iPad and keyboard with a soft pencil eraser (a drafting-film eraser is ideal). Ensure that the contacts on both iPad and keyboard are dirt and dust free - and all debris from the eraser have been removed - before reconnecting the two.
If you can't connect your iPad to your Magic Keyboard, Smart Keyboard, or Smart Keyboard Folio - Apple Support