Yep, you missed a step. You should have gotten an output similar to the following from flyctl certs create:
lillian ~ % fly certs create mynewdomain.com -a devplayground
You are creating a certificate for mynewdomain.com
We are using lets_encrypt for this certificate.
You can direct traffic to mynewdomain.com by:
1: Adding an A record to your DNS service which reads
A @ 18.104.22.168
You can validate your ownership of mynewdomain.com by:
2: Adding an AAAA record to your DNS service which reads:
AAAA @ 2a09:8280:1::3:c776
The AAAA record is required to verify the TLS certificate. If you add that to your Namecheap dashboard, it should work after that.