In order for students to use be able to resubmit answers, teachers need to enable this option under their advanced settings. To do that:
Log into Nearpod.com
Select the Silhouette icon (the head in a gray circle or your picture)
Click on the gear wheel next to your name
From account view, select the advanced settings tab and push the slider to on, as shown here:
Keep in mind that after you enable this setting, you will need to launch a new lesson code to see it take effect.