I can't pair Apple Pencil 1 to iPad 9 using usb-c to lightning cable

Some badly designed third-party covers are not Pencil-friendly - and will physically prevent direct connection of the Pencil with the iPad.

You'll not be able to pair/charge your first generation Pencil using a Lighting to USB-C cable. The charging adapters supplied with recent first generation Pencil's are not intended for connection with iPad models that feature a Lightning port.

The problem is not just charging, but Bluetooth pairing of the Pencil to your iPad.

With your iPad in a completely sealed-up/waterproof case, that is hard to renove or get into, pairing of your Pencil to your iPad will continue to be difficult and a frustrating experience for you.

Maybe you should consider a non-Apple, third party stylus that does NOT need any direct bluetooth pairing and can chsrge directly from a USB-C cable connected to your Apple power/charging adapter block, instead and still supports palm rejection.

There is the Logitech Crayon from Logitech

https://www.logitech.com/en-us/products/ipad-accessories/crayon-ipad-digital-pencil.914-000051.html

and other stylus options, like this one below made by Adonit

Here is one my Wife uses with her base model 8th gen iPad.

https://www.adonit.net/collections/styluses-for-ios/products/neo-duo

Best of Luck to You!