Hi Sankalp,
We can add an on-demand workflow on the Qualify stage to show the error message.
First create a real-time workflow for Lead, and check As an on-demand process.
Then add multiple conditional branches, each condition is if a mandatory field does not contain data, the action of each branch is Stop workflow, and the status is Canceled, then Set Properties to write the error of each mandatory field.
Let me take an example. I have two mandatory fields need1 and need2. So I create two conditional branches, one condition is If need1 does not contain data, the other condition is If need2 does not contain data. One message of Step Workflow is "You have to update the need1 field", the other message of Step Workflow is "You have to update the need2 field".





Then edit the BPF, add a this workflow into Qualify stage. The trigger is Stage Exist.

This way, when user click the Qualify button, and if there is any mandatory field does not contain data, user will see the error message we set.