According to the induced fit model, the active site is flexible. Does it mean that any substrate can attach to this flexible active site? If not, then explain.

No, not any substrate can attach to a flexible active site. According to the induced fit model, the enzyme’s active site changes shape upon binding to the substrate, but it still requires a specific fit. The flexibility allows the enzyme to better accommodate the substrate, but only substrates that have complementary shapes and chemical properties will bind effectively.