44AD(4) Applicabilty

Yes for ay 20-21 you can opt for 44AD there is no such restrictions regarding opting for presumptive taxation as you have to maintain profit of 8% of turnover, if your profit is equal to or more than 8% of turnover then audit is mandatory even if turnover is less than 2 crores

Your answer is not clear....
firstly you said can opt
secondly you said audit is required

Yes, there was a mistake..
In case the person is covered under presumptive taxation scheme of Income tax act than the assessee is required to conduct tax audit if the income from business is less than 8% of the total turnover in case of business assessee or less than 50% of the total receipts in case of person carrying professions. but you can opt for presumptive taxation

But there is condition that if exit from 44AD than can't opt for 44AD for next five years... Audit is compulsory except under section 44AD(5)

You can opt for section 44ad but if you opt for 44ad you have to to opted for continuously five years if you withdraw you cannot take it for the next five years