Why can't I set my ticket to solved?

Question

Why can't I change my ticket status to solved? I need to solve the customer ticket, but one of the following error messages appears:

"Ticket field name" needed

Assignee is required when solving a ticket

Answer

You can't solve a ticket unless the ticket has:

An Assignee

All mandatory fields contain a value

To solve this behavior:

Refresh the page

Verify these two elements

Try again

Note: The error message lengthens if more required fields are left blank.

For a similar issue, see: The Solved button is grayed out on my messaging ticket.