There are a few different possibilities. Kakuje provides one option, but there are other potential issues. Bottom line, what is the exact response that you get when you put the 6-character TB code in to geocaching.com/track?
If it says that the trackable doesn't exist, then it could be an error with the code. Some characters may be hard to read, and there are some character that are not used -- for example, unless it is part of a custom coin code, like "OC" for Oak Coins," the letter O is not used for TB codes -- try using the number 0 instead. Similarly, I should almost never be used, try 1 instead. Other letters can simply be hard to read because they are worn away or were not well printed in the first place -- a Z might look like 2 or vice versa, an 8 can be mistaken for a 3 or a B, a Q might look like an O.
Or, it may not be trackable on geocaching.com. (If the code is more or less than six characters, it won't be.) There are some competing websites that sell products that look similar to travel bugs, but are not trackable on geocaching.com, like pathtags or geokrety. It should say something to the effect of "Log me on geocaching.com" -- if it does not, then you need to log it elsewhere.
If the trackable isn't activated, then it may be yours to keep and activate it under your account.
Finally, some travel bugs may have been moved into a cacher's collection and then released into the wild anyway. You'll know this is the case if you only have the option to discover the trackable and not to grab it from the current holder. If this happens, contact the trackable's owner, let them know you have it, and work with them to see if they will release it from their collection so you can move it along.