As you play through the main story in Season of the Construct, you will come across the Drums of the Vault quest which tasks you with upgrading the Seneschal. If you’re having trouble using the upgrade at the brazier, I have just the guide for you in Season 3.
How to Upgrade the Seneschal at the Brazier in Diablo 4 – Fix
To fix the “Drums of the Vault” quest, you need to restart Diablo 4 and then return to your task of upgrading the Seneschal at the brazier. Upon success, you can interact with the brazier and place the Pearl of Warding into the fire.