I found a mistake in the document that I attached so I'm emailing you to let you know
「添付した資料の中身に誤りが見つかったので、お知らせします」は英語で、I found a mistake in the document that I attached so I'm emailing you to let you know になります。
添付した- attached (過去形)
資料 - document
誤り- mistake . 日本人がいうミスは多分ミステイクの略だと思います。
見つけた - found (過去形)
お知らせする- to inform / to let someone know something
メールの雛形
Dear ...
I found a mistake in the document that I attached (yesterday, a few moments ago...) so I am emailing you to let you know.
I have attached an updated version of the document to this email.
My (sincerest) apologies
Kind regards
( First name/ full name)