Can I use a 20W adapter to charge my iPad Air 2?

Yes.

With an appropriate USB-C to Lightning charging cable, the Apple USB-C 20W Power Adapter is suitable for charging your iPad Air2.

• USB-C to Lightning Cable (1m)

https://store.apple.com/uk/xc/product/MM0A3ZM/A

Hi,

The following articles should be able to further assist:

• Important safety information for iPad
• Power & Cables - iPad Accessories - Apple
• About Apple USB power adapters
• Using Apple power adapters, cables, and duckheads with Apple products

Yes, BUT such a adapter WILL NOT ever fast charge that iPad.

Fast charging is NOT supported in an 8-year old iPad Air 2.