I am using version 11.3 of Mathematica and when I make my notebook files I make sure to convert specific cells into text and deselect "evaluatable", yet when I save and reopen the file, the text cell is an input cell. How can I stop this?
It's not possible to tell for sure without knowing what you are doing exactly, but I suspect that you aren't really creating text cells.
This is how it is normally done:
Position the input cursor between cells (not within a cell). It will look like a horizontal line (not vertical).
Now choose Format -> Style -> Text from the menu. I believe the shortcut is Command-7 on Mac and Alt-7 on other platforms. This will create a Text cell.
Do not change cell properties like "Evaluatable". It is not necessary.