Seems pretty simple. Did you make an offer to buy this home or not? If not yet, do you want the house or not? Info online is not 100% accurate.
So you're standing in front of the house and there is no garage. Okay. Either you agree to move forward and make an offer or you move on to another property.
As people have said info on line isn't alwats accurate. I can also tell you trulia and zillow take info from the broker and then make further errors. You will save yourself time working with and agent or using realtor.com which has more accurate info with less transcription errors
Information in mls is subject to errors and omissions. Your agent can inform the listing agent. If you are looking for a home with a garage look at other homes. If you like the home and want to buy it make sure it is priced right and make an offer. Good thing you noticed before making an offer.
What do you want? If you want a garage then don't buy the house or see if you can build a garage. Obviously there is a mistake, these happen. Unless there is a garage available elsewhere or the garage was converted. If you like the house you will want to get an understanding as to what happened to the garage. Was it stolen? LOL. Was it removed? Was it converted? Were there permits? Or did it ever exist and was simply a typo?