I've written about the quirky behaviour of documents in Page Layout mode, when Master Object options become unavailable. A reader is asking how to choose between Page Layout and Word Processing modes.
When you open a new document, template chooser should open too prompting you to choose 'blank' from either Word Processing or Page layout. Click on 'blank' under Word Processing.
If template chooser isn't set to open with every new document, under File menu choose 'New from Template Chooser', or type Shift+Command+N.
If you've already done much of your work, it might be wiser to put up with that quirk. If not, open a new document in Word Processing mode and copy-paste your work into it.
Life is not fair!