How to pronounce "can't"?
We usually say "can" with sound "æ" between.
But I heard that the word "can't" we should pronounce with "a" (like letter "r"). Is it true?
Could someone from native speakers explain me that?

Question

Answer

Online dictionaries are a wonderful resource.

I recommend the Oxford Learner's Dictionaries website.
https://www.oxfordlearnersdictionaries.com/definition/english

As Ivan mentioned, the word "can't" has two pronunciations.

https://www.oxfordlearnersdictionaries.com/definition/english/can-t?q=can%27t

can’t short form of cannot
BrE /kɑːnt/ ; NAmE /kænt/

The Oxford dictionary gives strong and weak pronunciation forms when they exist.

https://www.oxfordlearnersdictionaries.com/definition/english/can1?q=can
can1 modal verb
BrE /kən/ ; NAmE /kən/ ; 
BrE strong form /kæn/ ; NAmE strong form /kæn/

Answer

A word of caution for non-native speakers - unless you have mastered the difference between /kɑːnt/ and /kʌnt/ , thread very carefully  :)

Answer

It’s pronounced “cahnt” in Australian, British, New Zealand, Irish and South African English and “cænt” in American and Canadian English. So, both pronunciations are correct and it’s just one’s personal preference whether to pronounce it “cahnt” or “cænt”.