Sure! Here are the underlined parts that should be corrected in each sentence:

21. My sister is very beautiful **with fair wavy hair and slim build.**
**Correction:** "with fair wavy hair and a slim build."
**Explanation:** The article "a" is needed before "slim build" to make the phrase grammatically correct.

22. One of the most serious **problem** now is the spread of COVID-19.
**Correction:** "problems"
**Explanation:** "One of the" should be followed by the plural noun "problems" to indicate that it is one among many.

23. If I were you, I **didn’t** buy that expensive car.
**Correction:** "wouldn't"
**Explanation:** In this hypothetical situation, the correct form should be "If I were you, I wouldn't buy that expensive car."

Let me know if you need any further assistance!