Why won’t Apple Pencil 1st work with 11” iPad Pro

Here is the compatibility list from Apple

https://support.apple.com/en-us/HT211029

You need 2nd gen for the 11”.

Because both versions of the Pencil charge and pair differently.

You need the 2nd gen Pencil for any newer 11 and 12.9 inch iPad Pro models, as well as for the 2020 iPad Air 4.

Sorry.

Why?

The first and second generation Pencil use an entirely different pairing and charging interface. The second generation Pencil can only be used with iPad models that have the magnetic/inductive dock that is exclusively designed for the Pencil. Charging is controlled via the iPad magnetic dock; the second generation Pencil cannot be charged using a charging mat or Qi charging solutions.

The first generation Apple Pencil uses the physical Lightning port for both pairing and charging the first generation Pencil. This pencil can only be used with compatible iPad models that have a Lightning connector.

In summary, the Apple Pencil models are not interchangeable - and can only be used with the iPad that has appropriate internal hardware.

Here is the definitive compatibility list for Apple Pencil and iPad:

Apple Pencil (2nd generation)

You can use Apple Pencil (2nd generation) with these iPad models: 

• iPad mini (6th generation)
• iPad Air (4th generation)
• iPad Pro 12.9-inch (3rd generation) and later
• iPad Pro 11-inch (1st generation) and later

Apple Pencil (1st generation)

You can use Apple Pencil (1st generation) with these iPad models:

• iPad (9th generation)
• iPad (8th generation)
• iPad mini (5th generation)
• iPad (7th generation)
• iPad (6th generation)
• iPad Air (3rd generation)
• iPad Pro 12.9-inch (1st or 2nd generation)
• iPad Pro 10.5-inch
• iPad Pro 9.7-inch

Not sure which iPad you have? Identify your iPad model.