I cannot play online

This is due to the fact that there are so many players on right now. Apparently, there are server caps and this results in not being able to find a match online in any of the gamemodes. I can only recommend being stubborn and constantly throwing yourself at the portal and constantly switching the gamemodes in hope of finding a server. The most two popular gamemodes is Mixed and Turf Takeover, so I recommend trying the other less popular ones in hope of finding a game.

Since I've installed the game, tried once a week,  few times, and i couldn't play not even once. I played only Single player so far.

hi ! idk what game your talking about but if it's pvz heroes you need to change your phone or emulator to american english theres no other way to fix it sadly... and if it's pvz gw2 it's just the server are full try play in the morning, try other gamemodes or change the region on the multiplayer console the best chance to find a game is australia but 300ping so yeah not so fun lol