For the Animation projects, we have a total of 7 templates. Of the 7, 3 templates will allow you to set background images and 4 will allow you to set solid color backgrounds.

Here are the templates that will allow you to set and change background:

If you have selected a template that only allows you to set solid background colors, you will not be able to set animated backgrounds for them.
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You can choose to go back to the "Theme" section and select a template that supports having an animated background.