Hey there sunnibakchat,
Thanks for reaching out to the Apple Support Communities.
Just to clarify, are you clicking on the Send button, or the pop-up menu next to the Send button?
For more details, check out this link here: Write, send, and schedule emails in Mail on Mac - Apple Support
Send an email
In the Mail app
on your Mac, click the Send button
(or use the Touch Bar).
Schedule an email with Send Later
In the Mail app
on your Mac, click the pop-up menu next to the Send button
, then choose an option.
Emails you choose to send later appear in the Send Later mailbox in the sidebar.

If click on the Send button, can you provide us with a few more details on the pop-up box you're seeing?
Also, check out the information in the additional resource on the changes that came with Mail on macOS Ventura: What’s new in Mail on Mac? - Apple Support
Keep us updated on what you find.
Have a great day.