This door says Authorized Personnel Only.
The backrooms of grocery stores are usually for staff only.
That is wild! I can’t believe you went into a place marked "Do Not Enter" and then peed in there!
Standing and entering are both prohibited (禁止). If you see this sign, it's essentially saying do not enter or no entry.
The reading for this one is weird. 立 is たち and 入 is いり. It's like you're taking the words 立つ and 入れる and stuffing their hiragana parts back into the kanji (and changing it to いり in the case of 入れる). So, that's something you'll want to try to remember. 禁止 is normal, though, so at least you have that.