When you get this error message, it means that the font that the template designer designed the template with is not installed on your computer.
If you do not plan to use the font that came with the template you should still be able to use the template as it is with your own fonts.
Replacing the missing font with your own font
You will need to find the layer that uses the missing font and change the font to whichever you want.
Text layers that use the missing font is indicated in the Font Family selection of the Character panel with the "" symbols, e.g. "[OstrichSans]" as shown in the diagram above.
You can simply switch out the font here with one you already installed.
If you have the font installed already, but it's of a different Font Weight that the template uses, this error message will still pop up, so do take note of the Font Weight as well.
Locating and Installing the Font Used
When the font determines the look and feel of the template, you may want to install the fonts used by the template designer.
Some of our template artists will have put the links to download the font in the template description of their template's details page as shown:
You may be able to find them in the help document that comes with your template, such as in this PDF guide:
If you can't find the fonts, you can do a Google Search for them as well. Just copy down the name of the fonts in the error message and drop them into Google Search.
Also, take note of the Font Weights (e.g. "Medium", "Bold", "Light", etc.) that are being used by the template.
Here we search for "Font OstrichSans":
Do remember that not all fonts are freely distributed. When the template uses the font, it doesn't automatically mean the font's author has granted the right for you to use it as well.
The fonts are their authors' property, and can either be a paid product, freeware, shareware, demo versions or public domain. Check the indicated author's website for details, and contact him if in doubt. If no author/licence is indicated, that doesn't mean it's free.
After you have downloaded, you can proceed to install the fonts.
After you have installed the fonts, close your After Effects Project file without saving it. Now reopen it.
The error message with regards to the missing font should disappear and your template project file will load with the correct font now.
Video Tutorial on Adobe Support
There's a video tutorial on the Adobe After Effects' Learn & Support page that also teaches you how to Easily find missing footage, fonts, and effects. Click on the link below to watch the tutorial.