If you are using imap and you do not see a Trash folder, you have to sign into your email provider's web mail service to undelete the email.
You may be able to change the behavior and now have deleted messages stored on your computer. You are using quite an old macOS and I don't know if you have the same settings. On my Sequoia Mac if you go to Settings (preferences) > Accounts > Mailbox behavior > uncheck "Keep deleted mail on server"
it will now start keeping deleted mail on your computer itself and a Trash folder will appear in your sidebar. This will not, of course, help you with the already deleted mail. For that you will have to sign into your email provider's web based application.