After updating to or installing v102 DNSONLY, the root password cannot be changed through WHM as the menu entry has been removed.
This is a known issue and one our developers are looking into.
We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-40296. Follow this article to receive an email notification when a solution is published in the product.
You can still change the root password through SSH as outlined on the article below.