hanamaru
Say you can do something
できます says you 'can do' something, with the thing marked by が.
JLPTN5
Noun + ができます~ ga dekimasu
someone can do [something]
The thing you are able to do is the grammatical subject here, so it takes が, not the object marker. This is a common trap, because in English the same idea uses a direct object.
Tips and Common Traps
Hana· Key point
Great for saying what you're good at or which languages you speak — like 日本語ができます (I can speak Japanese).
Maru· Common trap
The noun before が must be a skill or language, not a verb — say 料理ができます, not 料理するができます.