A simple but common error is to use a number and the plural form of a noun to form a compound adjective:
We took a two weeks holiday. ✗
Let’s take a ten minutes break. ✗
I found a fifty dollars note. ✗
These must use the singular forms of the nouns (week, minute, dollar):
We took a two-week holiday. ✓
Let’s take a ten-minute break. ✓
I found a fifty-dollar note. ✓
Note: hyphens (-) are needed.