"I am going to buy a watch tomorrow."
If you want to be more specific with the translation, you can say:
- 我明天會買一個手錶 (Complete)
- 明天我會買一個手錶 (Complete)
Note: Duolingo misses some translations with time and location, so I include extra translations. As long as time is used in setting, you can use it interchangeably in a sentence.
When the quantity is not specified a classifier is not needed either. The grammatical use of indefinite article cannot be translated into Chinese. Duo's translation is fine. Certainly 明天我会买只手表、明天我会买一只手表 are also acceptable translations.
Interesting. So there's probably many other questions in the course where most of us are using classifiers but which would be just as correct without them?
Then again, the indefinite article does have the effect of specifying a quantity...