Once
One Time
noun
One universal event can only happen once. It's too rare to happen more than that.
いっぺん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. Just watch out because へん becomes ぺん here.
一遍だけでいいから、キャビアを食べてみたかったの。
I wanted to eat caviar but one time was enough.