See #2 below.
From
https://store.n2acards.com/faq#question-149
N2A Nook Tablet Card - What do I need to know?
1. In order to boot into Android, your Nook Tablet must first be completely powered down. You do this by holding down the power button until the screen prompts you to "Power off".
2. If you want to use the Nook Tablet as a normal ereader, turn off the tablet as noted above, but this time choose "Reboot". Your device will reboot into Nook Tablet mode and bypass the card. You do not need to remove the card from the device
If you have a problem, go to the N2A website Help section.