Return key doesn’t work in IntelliJ IDEA code editor or it’s doesn’t work in code completion popup.
The Keymap configuration is wrong. You probably accidentally assigned the Enter key to the another action.
The easiest method to fix the problem is to reset Keymap to defaults. Go to the File->Settings->Keymap and then choose Restore Defaults.
The another solution of the problem is to revert previously made changes – one by one. Go to the File->Settings->Keymap . All chances you have made are marked on a different colour like it’s shown on the image below. Review all marked assignments and fix them manually.