Hi Steve,
There is a "Textbox tool" under the Comment tool on the tool panel at right.
Open a PDF in the application and click on the "Comment" in the tool panel at right.

You will find the Text Box tool there. Check the screenshot below:

If this is not what you are referring to, please provide us more information about the tool.
Share the exact steps you used to open that tool.
If possible, it would be helpful if you can share a screenshot of the tool that you are referring to.
Also, please update the application to the latest version available.
Launch the application and go to Help > Check for updates.
You may also refer this help link Release Notes | Adobe Acrobat, Reader to check the latest updates available.
Regards,
Meenakshi