In another sentence: "You were trying to drink water", Duolingo gave the translation: "Usted trataba de beber agua". Why is "Su primo tocaba al gato" not correct in this case?

tocaba should be correct

I know that "a el" turns into "al". Is this a very strict rule? For example, imagine someone is speaking very slowly (as if making up a story on the spot). He says "Su primo estaba tocando a..." and then he thinks that he could say "la lámpara" but changes his mind and wants to say "el gato". Would he forge ahead and say the "el"? Basically, how wrong is it to say "a el"? Thanks!

su prima estaba tocando al gato - so I can not have a female cousin, which is all I have