Some basic troubleshooting steps you can try, in order:
# Check your router, it could be an issue with your WiFi network. Try it on another WiFi network and see if the error still comes up.
# Clear Google Play’s cache:
## Open Settings
## Click on “Apps” or “Applications Manager”
## Find Google Play in the list and click it
## Click Clear Cache
## Try downloading an app
# Clear Google Play Services’s cache:
## Open Settings
## Click on “Apps” or “Applications Manager”
## Find Google Play Services (looks like a puzzle piece) in the list and click it
## Click Clear Cache
## Try downloading an app?
# Delete Google Play’s data: This will probably sign you out of Google Play, so you’ll need to sign in again…
## Open Settings
## Click on “Apps” or “Applications Manager”
## Find Google Play in the list and click it
## Click Clear Data
Hopefully one of those options works for you. Right now I’d probably lean towards some WiFi issue rather than a problem with your phone…
Luckily you have unlimited data anyways, so there shouldn’t be a big deal if you do need to just use mobile data :)