When you're attaching or sending a Google Docs file in an email, the Send button doesn't become active until you've entered a recipient email and message body.
If you've done this and still can't use the Send button, make sure you're using a valid Google email address. While you can sign up for a Google account using almost any email address, you'll need a Gmail account, or Google Workspace email, in order to send an email directly from a Google Doc.
For example, I have a Google account under a "yahoo.com" email address. I can't send email attachments while logged in with this email account. I have to use my "gmail.com" address instead.