I found a very easy and simple solution.
I hope it will help you.
Generate SSL from any ssl provider (zerossl.com if free)
When you download them, there will be 3 files key, certificate, CA Bunddle.

Now go to your aaPanel Click Websites > Conf of website > SSL
You will see box like this

Click on disable certificate,
then go to Other certificate tab

There will be 2 boxes. Empty these 2 boxes, Paste your downloaded private key in private key box
in Certificate (CRT/PEM) box: first paste certificate.crt then after this post ca_bundle in same box

All is done. you are now done.