Can't play online with 2 local users with a single PSplus account

I am trying to play this game online with two local users at my PS4. Only one of the users has PSplus (main PS4 account). However, the game will not let the second user join the first user's game in Rocket League. It says the second user needs a PSplus account as well.
My research on PS4 says that multiple accounts can use PSplus as long as the main account has PSplus and the PS4 is registered as the main under his name.
What am I doing wrong?
Best Answer
Well, in our case my girlfriend had to create a PlayStation account (not plus) to be able to join. A simple local account in the PS4 will not work. We definitely played online quite a bit and I am the only one with the PSPlus subscription.
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Can 2 people play on the same PS Plus account at the same time?
So long as one account on the PS4 or PS5 has PS Plus and is set as the primary account for that console, anyone who signs into that console with another PlayStation Network account will be able to share many of those benefits and play any digital games associated with it.Do both users have to have PlayStation Plus to play online?
In order to play most PS4 and PS5 games in online multiplayer, you need to have a PlayStation Plus subscription.Do you need PS Plus to play it takes two split screen?
Yes, both players will need a PS Plus/Xbox Live Gold subscription to play together online.Can - Mother Sky (1970) [HQ]
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