First, avoid using text boxes to insert sophisticated formatted text. Text boxes are drawing objects which do not interact well with the main text flow of the document. Text boxes are rather limited in their formatting flexibility. They are great for adding short labels to graphics but not for an elaborate text such as the one you exhibit.
Prefer frames when you have a “secondary flow” (a sequence of paragraphs with complex formatting where paragraph and character styles come in handy, what you can’t do in text boxes.
Now, your problem.
When you gave your text box wrap attribute In Background, your text box was sent (warped ;-)) to the background, one layer behind text. It was removed from the constraints of text flow and ordinary text could then be typeset over it. After that, it is nearly impossible to click on some non transparent element of the text box because of the overlaying text.
Use the navigator, Drawing objects section, and double click on the text box object (it has a name like Shapen where n is a number) to select it. You can now
Wrap>any wrap option.
If you realise that In background was an error, you can
Z to undo Apply attributes (this is what is shown in the
Edit menu) provided you have not too many actions after the error. In which case you must undo these first.
Z from the keyboard works also here with LO 18.104.22.168 under Fedora 32, Plasma desktop.
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