Cannot save work or open documents

[Capitan]

When I have written a letter and want to save it for use later on I am unable to do so and certain letters I have already written I am unable to open

[Hagar Delest]

Hi and welcome to the forum!

Where are saved your letters? You are the one that tells the application where they are saved.
You can use the menu File > Recent documents to access them.

[RoryOF]

While not an ideal location, as it gets too crowded, you should always be able to Save to your Desktop. If you can do this, it indicates that you may not have permissions to Save to your other locations, which probably requires you to change your Windows Defender settings.