Here's an error correction worksheet on modal verbs. It covers the mistakes made in forms or meanings of modal verbs. They are typical mistakes made with modal verbs included in the pre-intermediate level of English. A suggested answer key is also given in the teacher's copy.
Most of the mistakes in this worksheet are made in forming modal verbs. Modals are pretty easy to form; however, students make punctuation errors a lot. This worksheet can help them realise these kinds of mistakes. Besides, students sometimes forget to use the bare infinitive form and try to conjugate the verb after the modal verb according to the subject of the sentence.
I also suggest doing peer-correction activities after this worksheet, which will help make your students better learners. After a writing activity, it would be great to ask students correct their partners' texts.