As of now, Coinbase does not offer the option to buy UBT. UBT is not a very popular coin, so it is not surprising that it is not available on Coinbase. However, there are other exchanges that do offer UBT, so if you really want to buy it, you will have to look elsewhere. There are a few things to keep in mind when buying UBT though.
First of all, make sure that the exchange you are using is reputable. There are many scams out there, so you need to be careful. Secondly, UBT is not a very stable coin, so its price can fluctuate a lot. Make sure you know what you are doing before investing any money.