I can not type in Messages or other DM chats

For some specific users, the type bar may not appear when in Twitter DMs, Instagram DMs or Messages accounts.

If it doesn't show for you then, from v5.0.7 or later, there is a 'Fallback Composer' which you can enable/disable from the 'Global Preferences' from the side menu.

This will present an alternative method of creating messages for your DMs.