Hi Jill,
Thanks for posting in the community. We are happy to help you.
According to your description, the Outlook dialog box does appear and allows you to send an email with an attachment, but the recipient does not receive the email and you do not see the email from Sent Items or Outbox. To troubleshoot the problem, I suggest you manually send an email to this recipient and then check again.
Also, I suggest you repair Office via Windows Settings> "Apps & features" (or Apps> Installed apps)> find and select the Office product> Modify> choose Online Repair. After that, please test again.
If you can manually send the email successfully, but the problem still occurs after repair, I would like to gather more information about the situation.
1. Did you check the email in the New Outlook? I suggest you type "Outlook" in the Windows Search bar and check if you can see two Outlook applications (Outlook (new) and Outlook). If you can see two Outlook applications, please run Outlook, check the email, and check the account connectivity in the Status Bar.
2. Check Other Folders
Check other folders such as Drafts to see if the email is stuck there.
3. If you have added multiple accounts in Outlook, I suggest you check other accounts' Outbox or Sent Items.
4. Are your colleagues experiencing the same problems?
5. Please provide your Office version and Product Information.
Click File> Office Account, please take a full screenshot including Product Information and About Outlook. You can click "Insert Image" to upload it.

We look forward to your response. Thanks for your cooperation.
Sincerely,
George | Microsoft Community Moderator