DIAGNOSIS The printer and your phone need to be on the same Wi-Fi network. If one is on the 2.4GHz band and the other on 5GHz, or if you have a "guest" network, they can't talk.
FIX 1) Check your printer's network: press the Wireless button on the control panel. It should show your network name. 2) On your phone, go to Wi-Fi settings and make sure you're connected to the exact same network (not a similarly named one). 3) Close the HP Smart app completely and reopen it. 4) In the app, tap the "+" to add a printer. It should search and find it. If it doesn't, the app might need to connect directly: hold the Wi-Fi button on the printer for 3 seconds until the light blinks, then try the app's "direct connection" option.
IF THAT DIDN'T WORK Your router's firewall or settings might be blocking the discovery protocol. Try temporarily disabling your mobile data so your phone is forced to use only Wi-Fi, then try again.
HEADS UP Many phones have a "private Wi-Fi address" setting (like on iPhones) that can interfere. For testing, you can turn that off for your home network in your phone's Wi-Fi settings, then turn it back on later for security.