Sweet Potato has an onboard bootloader that should display the boot screen without any software loaded. Make sure the boot switch is in the SPI NOR position: Where can I find information about how to use the boot switch on the Sweet Potato?
You should use the +arm64 image since this is not the V1 which doesn’t have the onboard bootloader. If you have the switch in the wrong position (MMC) and you’re using the V1 image, it will try to use the V1 bootloader on the V2.