Contains characters that do not exist

[CWBillow]

When saving my document I get a warning that the file "Contains characters that do not exist in Code page 1252, ANSI Latin 1"

How do I fix this?

[IanOfYork]

That is generally due to pasting in text from somewhere else – a web page or document that includes one or more characters that cannot be represented by the plain text encoding. They can be obvious things, like emojis, or less obvious ones like currency symbols and curly quote marks.
You have two options:

1. Hunt down the offending characters and delete them or replace them with something plain
2. Save the file using another encoding. UTF-8 is probably the best option

If this happens a lot, you may want to set the default file encoding to UTF-8. Just about all web pages are UTF-8 these days.
Saving plain text as UTF-8 will not make for larger files, but UTF-8 will allow for (and preserve) just about any character.