I want to offer a massage service, to make some extra money. Is this type of service popular and would I get clients?

Yes. But they will think that yo uare a different kind of massage person and you will be unpleasantly surprised half the time.

str you a trained masseuse?

Most states require you to be licensed, which means at least 6 months of training and passing an exam.

You have to be licensed by the state to do it. AND have insurance, particularly liability insurance.
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Licensing. Most states regulate the massage therapy profession. Depending on the state, this could be in the form of a license, registration or certification. ... Licensing is the strictest form of professional regulation, making it illegal for anyone to work as a massage therapist unless he or she has a license.

Massage Therapy Credentials | American Massage Therapy ...
https://www.amtamassage.org/findamassage/credential.html

Where are you planning to offer this service? What kinds of qualifications do you have, and how will you be advertising your business? Massage services can be popular, but if you aren't a qualified therapist and are just planning to do this because somebody told you that you give good massages once, you are less likely to be successful. Unless you will be set up in an area to attract walk by traffic, how you advertise to draw clientele will matter a lot. Personally, I wouldn't go for a massage with just some random person who wasn't affiliated with a spa/clinic/service of some kind unless I knew them personally.