Samsung Pay and Gift Cards

[Mojave]

I have the Galaxy S9+ on Verizon. I have been trying to add the Google Play card to Samsung Pay. I have opened the SP app and tapped on "cards". You can add a credit or debit card.....gift card...and membership. I tap on gift cards and tap on "load". From there you can scroll down a list of gift cards. I found "Google Play" listed as a gift card and tapped on that. I then tried inputting the card number that I have scratched off from the back of the card. Tapped confirm and it then gives me an error that the information is invalid.

I purchased the Google Play card from Giant Eagle which is a major grocery store where I live. What am I doing wrong? I do have two credit cards that I use on SP with no issues. So why not the Google Play gift card? Thank you.