drop down menus blank when using S.C.A. provided forms

I’m using LibraOffice Calc. Version: Windows 10

Suddenly a red highlight around the frame of the cell has appeared when I paste info in Calc.
Followed by a red line across the page.
What is it?
How can I turn it off?

Btw, your question head line does not match your question at all…

You activated Change Tracking, probably by accidentally hitting Shift+Ctrl+C, disable with Shift+Ctrl+C again or menu Edit → Track Changes… → uncheck Record.