Questions & Answers about Das Üben macht Spaß.
Üben is normally a verb (to practise), but here it is used as a noun: das Üben = the practising / practice (as an activity).
In German, all nouns are capitalized, so when an infinitive verb is turned into a noun (a nominalized infinitive), it is written with a capital letter: üben → das Üben.
In Das Üben macht Spaß, Üben is a noun.
The verb of the sentence is macht.
You can test this by replacing das Üben with another clear noun: Die Musik / Das Lernen macht Spaß – the structure is the same.
The article das shows that Üben is being treated as a noun (the practising).
You can leave it out in everyday speech and say Üben macht Spaß; that’s also correct and quite common.
With das, it sounds a tiny bit more like “the act of practising (in general) is fun,” but the difference is small.