The key answer of exercise Grammar 1
The key to exercise solutions in chapter 08 - Language focus 1 for the textbook Cutting Edge Upper-Intermediate - Student's Book with authors Sarah Cunningham, Peter Moor and Jonathan Bygrave from Pearson Education
Question
Look at the website. Why is can correct in 1 but not 2?
Answer
Because in 2, there is already another modal verb, should. Only one modal verb can appear in a clause.