How to Add a Page on Tumblr if Your Theme Doesn't Support Itby Alan Sembera
When Tumblr gives you an error while adding a new page to your blog, you can fix the problem by simply creating your own page link in your theme's HTML code. This problem is a result of your theme's lack of support for page links -- you can still create a new page from your blog's Customize page, but you have to add the links yourself.
Click "Customize" on your Tumblr dashboard to access your theme settings.
Click "Add a Page" in the Customize pane to open the Add a Page panel. Although your theme may not support adding page links, you can still create the page from this panel.
Enter a page title, and then enter your page contents in the lower portion of the panel.
Replace "url" at the end of the Tumblr URL in the panel with a word that describes your page contents. For example, if you're creating an About page, change the URL to "yourusername.tumblr.com/about". (Use your own Tumblr username in place of "yourusername".) Copy this URL for later, when you create the page link.
Switch off the toggle next to "Show a Link to This Page" to avoid getting the error message.
Click the "Save" button to create the page, and then click the back button in the Add a Page panel to return to your theme settings.
Click the line inside the Description box at the place you want to insert your page link, and then enter the following line of code on a new line:
<a href="PageURL">Page Title</a>
Replace "PageURL" with the page URL you copied earlier, and replace "Page Title" with the word you want to use as your link in the sidebar. If you're creating a link to your About page, for example, the code should look something like:
<a href="yourusername.tumvlr.com/about">About Me</a>
Click "Save" and then "Exit" when done.