That error message displays when there is some technical problem with one or more recipient email addresses or the punctuation between addresses (in the "To", "Cc" or "Bcc" field). If you have multiple recipients, it may take some time to identify the error.
It's often difficult to analyze email addresses in the small field on the Yahoo Mail web page. I would suggest copying those addresses (highlight the entire field, then right-click+Copy) and pasting into a blank text editor such as Notepad (right-click+Paste).
You can then review the list of recipient addresses more easily. You may wish to insert line breaks, so you can look at each address on a separate line.
Some things to check:
(1) Make sure each email address is a fully qualified address (with user name, followed by "@" sign, and then domain name - no blank spaces in the address).
(2) If there are quotation marks around any address, make sure there is exactly one completed set (no more, no less) - with one quote mark at the start of the address, and one mark at the end of that address (like this: "firstname.lastname@example.org")
(3) If a recipient is listed by name as well as address, make sure the email address is enclosed in angle brackets, with any quote marks surroudning the combined name & address (like this: "John Doe ")
(4) Make sure there is just one comma or just one semi-colon (and no other punctuation mark) between each address in the list of recipients.
(5) Make sure there are no extraneous characters in the recipient list. Every character should be part of an email address (or combined name+email), or a comma/semi-colon separating the addresses.