First: Make sure that you've added a counterpart and that you've filled out and saved their contact information.
Second: Check if there are any changes or edits you've made that need to be saved. If there are, the send/sign button will be replaced with a blue "Save"-button.
Third: See if there are any required fields that need to be filled in. Required fields are marked with blue arrows in the contract or in the right menu bar.
Fourth: Check if you have any contract sections that are empty. You will recognise them by the light orange colour. Fill them with content or remove them according to your needs.
Save your changes to make the "Send"- or "Sign"-button green again!