My phone is not charging in 20w charger

Do a hard-reset. Try using another apple certified cable, or use the old brick that came with your phone or any one that you have from a previous phone.

To do a hard reset follow these steps;

Force restart iPhone X, iPhone XS, iPhoneXR, iPhone 11, or iPhone 12. Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side button. When the Apple logo appears, release the button.

Hello,

you can follow this guide to troubleshoot your charging issues If your iPhone or iPod touch won't charge - Apple Support.

-Drake