Why is my Samsung Galaxy S5 telling to insert a SIM card?

[puddytat2tn]

Bought my phone privately and got it activated through Sprint, but it now keeps telling me to insert SIM card. Where do I get a SIM card and why do I need one? Phone seems to work fine, but message is annoying.

 

[B. Diddy]

Welcome to Android Central! In order to use 4G LTE data (i.e., accessing the faster internet speeds using the phone's signal and not wi-fi), you need a SIM card. Call Sprint about this.