How to transfer pending earnings / BAT rewards from old PC to new one?

If you mean for this current month, payouts are still ongoing. You can track the status at Ads Payout Status Update

If you’re saying payouts didn’t happen in like December or January, then you need to create a Rewards Support Ticket at https://support.brave.com/hc/en-us/requests/new?ticket_form_id=360001302431