Problems with the massacre license plates

Did you use a translater of some kind, as I have no idea what Massacre bottle caps or license plates are? The way my trophy is described is: Get 5 multi-kills as assault in Multiplayer.

So first question did you use the assault class when attempting your trophy? If not that might be the first clue.

And here's what I found on a trophy guide: 

Multi-kills in Battlefield 6 are basically acts of valor where you kill 5 enemies within a short time of one another (you can keep a combo going by getting a kill within around 6 or 7 seconds of the previous kill). You basically need to do this on 5 separate occasions.

Luckily you don't have to do it in one match, so you can work on this one over multiple games if you like, but you must choose the Assault class to do it. That said, you don't necessarily have to choose assault rifles and can choose a DMR or SMG if you so choose.

Now, in terms of strategies, we found that fighting a crowded chokehold from afar while using grenades and the assault rifle is a viable tactic - although we much preferred the DMR in this situation. This trophy can be made infinitely easier, though, by unlocking the grenade launchers, specifically the M320A1 HE (High Explosive Launcher) or the M320A1 THRM (Thermobaric Grenade Launcher).

Then, you should pepper a crowded point with the grenade launcher and/or grenades (both stun and frag) and then mopping up with some well placed shots. 

I hope this helps and happy trophyhunting!