How do I delete text?

I have been maintaining a simple text file for several years using Libre Office Write. I save it as a .odt file. Occasionally I create a .pdf file. Recently something has started to go wrong. I am sure it is my fault but I’m stuck and would be most grateful for a word of explanation.

I change a bit of text once a week perhaps. I can insert new text but it does not allow me to delete the old text. Neither the “Back Space” or the “Delete” key works. All that happens is that the cursor moves back or forward, leaving the unwanted text in place. Also a thin vertical line appears at the left hand side of the line, in the margin. I’ve tried highlighting the text and using “Cut” but the same happens. I’ve tried Libre Office Help but can’t find the answer. I am embarrassed to ask for help!

You probably have clicked or selected (menu Edit) Track Changes…?

Thanks for your help. It was actually in File/Properties/Security/Record Changes where I had set it to record. You pointed me in the right direction (settings).

Removing the Record Changes in Properties/Security has solved the problem. Thanks to Grantler for taking the time to help.