"I practice the piano."
練習 is a noun that has verbal properties, they are often classified as "suru verbs" in English resources, this means that you can use the noun as a verb if you add する at the end,
like 練習する means "to practice" but 練習 alone means "practice".
You can also add the particle for emphasis on the action into the noun like 練習をする means "I do practice"...
And you can also use the masu form of する which is します in the same manner as in the Duo sentence.