Make sure the Droid Razr is not a 4G, I believe it has to be a 3G version to work as a prepaid? I have a Droid Maxx and that doesn't do prepaid.
I say to make sure because if you happen to purchase a prepaid card it may not be refundable if the phone doesn't support it.
Based on Verizon's own website in the link under FAQ's #1 it seems to indicate 4G is excluded. Sorry for the bad news, unless things have changed? But the link is from 2015.