Text boxes

When you insert a Text box, it is already selected, or if not selected, you only need to click it once for selection, and the Inspector panel shows an Arrange tab. Click that. If all that you see is the following:

You are in a Page Layout format Pages document and just look below for the Lock button.

If you see this:

You are using a default Word Processing Pages document and you select Stay on Page, the Lock option will be available near the bottom of the Arrange panel. In both document types, I have set Text Wrap to None so the Text box does not repel other nearby objects.

In a word processing Pages document, you can only lock a text box via the arrange menu when you have set its Object Placement to Stay on Page.

Click on the text box and in the right Inspector panel, will be an Arrange tab. Click that and what I explained will be clear.