The five incorrect words are: thanks, engagements, they, attach and apologise.
1- Thanks you for informing me about the meeting
Sandra should have wrote "thank."
2- ...because of previous engagements
Engagement is usually used to refer to a formal agreement to get married. Here, Sandra could have chosen a better option, like "commitment."
3- I'll be away 12th-15th June to interview they.
In this case, instead of "they" the personal pronoun "them."
4- I'm attach some ideas the investment plan.
Here, "attache" should be in the continuous form, "attaching."
5- Please accept my apologise...
"Apologise" is a verb. This sentence calls for the noun form "apology."
