What is the error? it's summer sale time! all womens' sandals and sunglasses are half off through the end of august.

the word womens'

Explanation:

We use the possessive form -s to express belonging. In the text given the writer wants to convey a sense of belonging between sandals and sunglasses and women.

The rules of possessive -s state that in order to make the possessive form we add 's at the end of the noun except in the case of plural noun that end in S in which we just add the apostrophe. The word women is a plural noun but it does not end in S, so, if we want to make it possessive we have to add 's.

Instead of WOMENS' it should be WOMEN'S

not concrete, existing in thought or as an idea but not having a physical or concrete existence

they should gives us more chances to make friend and do stupid things