If anyone could add whatever silly tag popped into their minds, we'd have a lot of silly tags.
The Stack Exchange system takes some getting used to, and rep is used as a metric for how much you likely know about the system. People that don't yet understand how the system works need to be prohibited from making a mess.
This case is a great example. You clearly haven't taken the time to learn the SE system and how tags are supposed to work, yet tried to create one. Common sense ("I'm new here, so if something doesn't look right, I'll assume the problem is my understanding of it.") should be good enough to prevent that, but all too often people are arrogant ("I'm new here and don't understand how everything works and what the norms here are, but eh, I just want to get my problem solved so I'll do what is expedient for me. Screw the system and all these dweebs here. I'll probably never be back, so what do I care if I leave a path of destruction behind me?") and won't take the trouble to learn and do things carefully. Therefore users must have some minimum experience level, measured in rep, to do things that can damage the system. Looks like it worked.
Frankly, 300 rep seems like a really low bar for creating new tags. That should probably be 1000 or so.