Outlook users have been reporting an issue on various online communities regarding a “Cannot send this item” error that they encounter.
The “Cannot send this item” error in Microsoft Outlook started showing up for users on long email threads and when replying or forwarding emails with embedded hyperlinks point to website pages with long URLs.
To solve this issue, here are some fixes that you can implement,
1] Change the Email Format to Plain Text
One of the things that you can do to get rid of this error, is to change the email format from HTML to Plain Text Format.
Changing the format to text format will show the embedded links in text format.
To do so, make sure you’re not composing your email from the reading pane. If so, click the pop-out icon to create a separate window for your email.
Then, go to Format Text > More options > Message Format > Plain Text.
2] You can retry sending the mail 2 to 3 times. The mail sending will be successful automatically.
Please find more fixes for the same issue on the below link,
https://www.mrsharepoint.guru/cannot-send-this-item-outlook/
If the issue still persists, then you can use the webmail to send the mail,