Do Not Enter
No Entry, Off-Limits
noun, expression, の adjective
Standing and entering are both 禁止 (prohibited). If you see this sign, it's essentially saying do not enter or no entry.
The reading for 立入 is a bit strange, but it might help you to know that it can also be written 立ち入り. The 立ち is the same one you learned with 立ち飲み and the 入り is the reading you learned with 入り口. In our version of the word, the hiragana isn't visible (it's sort of pushed into the kanji), but the word is still pronounced the same way: たちいり.
And 禁止 doesn't have any weirdness going on, so at least you have that!
スーパーのバックヤードは立入禁止です。
The backroom of the grocery store is off-limits.
このドアには、関係者以外立入禁止って書かれているよ。
This door says Authorized Personnel Only.
立入禁止って聞くと入りたくなっちゃうのが人間でしょ？
When people hear that something is off-limits, they want to go in, don't they?