I found when I updated my own Asus bios that some field names changed between versions, or were removed. So if the saved config is trying to set a value for a field call "Mega Speed Boost", but the same field is now called "Uber Mega Speed Boost" in the new bios, it's probably failing.
If you had entered your own VAT number here, the order would have been updated to Ex VAT, and so you would be liable for import VAT, as it states. I think maybe you assumed the VAT was included on the invoice, so just check that to confirm.
@MickF Did your invoice show you paid VAT? PC Specialist has an Irish VAT Number, so can charge Irish VAT directly on the invoice. If you paid Irish VAT at the point of sale, you shouldn't have DPD looking for further charges.
Check the 6 pin is not a 6+2 pin connection, with the 2 pin tied back. Save you running a cable for no reason. But also check the existing cable is not a daisy chain. E.g. single cable from PSU, with two ends to it connected to GPU. You want one cable per port on the GPU.
You’ve connected the monitor to the GPU connection and not the one on the motherboard?
Have you checked that all cables are fully seated, same for RAM and GPU?