You need to choose a template as a valid template type for the document type of your pages. Suppose you are a Document Type page called "ContentPage" and you have added a new template called "NewTemplate".
By default, you cannot select a new template when editing the "ContentPage" pages. You will need to go to “Settings” in Umbraco and then expand the “Document Types” folder. You will see a list of all types of documents, select the one you want to use in the template ("ContentPage" in this example).
The first tab you will see is the Information tab. There is a checkbox "Allowed templates" in which you need to select a new template. Save the document type and return to the Content. Now you can select "NewTemplate" as a template on your "ContentPage" pages on the site!
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