You can attach a file to an email using the drive service. You can also attach a whole email conversation to a single message. First, open the Compose window and select a file. You can then drag and drop it into your email. To attach a file to an email, you must select the Drive link. The link is then displayed in your recipient's inbox.
See also - https://emailspedia.com/unable-to-attach-files-in-gmail-how-to-fix-attachment-error/