I booked a flight ticket through an external website. The ticket I received shows my last name as one word even though it's two words. (Instead of "Full Name" it says "Fullname"). Can this be a problem at check-in?
You're fine. I have a space in my last name, and it's probably 50/50 that the space makes it through on a ticket, list of participants in an event, insurance policy, or anything else in life. It's only on rare occasion (taking a standardized admissions test; receiving nuclear launch authorization) that the other party really cares about the name on their side precisely matching my legal name. For something like air travel, it's not an issue.
As far as I know, international air travel standards allow up to three errors in ticket's fields. Even if space removal counted as error, that's just one.
In practice, it seems pretty allowing, for example, I've flew with my first and last names swapped without problems.
UPD: I'm struggling to find any formal confirmations to my "three errors" factoid, unfortunately, so your mileage may vary. But there are confirmations that you may ask the airline to correct it, and if they refuse ask them to make a note on your ticket that such issue exists.