There are several ways to copy a link to an assignment to send it to students. Sadly, one of the most obvious ones doesn’t work.
If you right-click and copy the link from the Assignments area in the Feedback menu or the Module Admin area, this is a link to the inbox where you will mark the work, and will show students an error message.

To create a link that works, you have a couple of options.
For situations outside of Brightspace, once you have added the assignment link to the Assessments area in Content (using the Existing Activities menu), you can copy this link and it will work.

Alternatively, if you are creating an announcement or sending an email directly from Brightspace, use the link tool and select Assignments.

