I have mbstring enabled in php.ini ("php -m" in CMD shows mbstring in the list), but I'm still getting this error.
Requirements page for mPDF says that it should include mbregex as well. I don't know, maybe it's missing. How can I install it? Or it is not the problem?
My PHP version is 7.4.19.
UPDATE: I'm using MAMP as my local server. phpInfo says current configuration has both mbstring and mbregex.
I have successfully reproduced this behaviour with MAMP.
Everything runs correctly with official windows PHP builds from php.net, so I believe there is an error in MAMP PHP build, even though everything in phpinfo seems OK.
Try submitting a bug report with MAMP.