we have requirement as below
When user tries to convert a Lead, if Account lookup is not selected with an existing account (ie user has to create a new Account), then require that user creates a new Opportunity.
Can we use a validation rule that doesn't let user tick the "Do not create a new opportunity upon conversion." checkbox if new account is created?? or we need to this using lead trigger?
Your best bet is to write a Visualforce page to override the Lead Conversion screen, and then put all your rules into that. The page can do whatever you want it to do based on what fields on the Lead are filled.
I would make the page let the user choose an existing Account for the newly-created Contact OR (create a new Account AND create a new Opportunity). This can be achieved with conditional rendering in the Visualforce page.
I am not suggesting a Lightning Component because the latest org I looked at only allowed Visualforce override for the Convert Lead action.
