Issue with using WWW in URL, Internal Server Error

I am not able to access website using www prefix (
But It’s working without www prefix(
I tried to configure the settings according to this post but it’s not working.
can anyone give me hint or suggestion to fix this issue.

check below link:

If you want your site to only be accessible by specifically typing “”, you’ll add “www” to the record name. If you’re fine with “”, you can leave it blank.

If you can’t decide between “www” and no ”www”, some DNS management systems offer a simple trick to bypass the restriction. Entering the “@” symbol in the Name input instead of leaving it blank allows you to “copy” another record’s value and data. This means the other record will refer to the value of the base form before resolving. If you want to stop the “www” / no “www” division, then simply set the DNS record to have the “@” value, then sync these changes to the base record, too.