The first thing to note is that a Google Doc template isn't just any document in your Drive account. It has to be designated as a template, and instructions on how to do that can be found here.

Also keep in mind that your placeholder variables need to be created like {{this}} - with two opening/closing brackets on either side. There also can't be any spaces in your placeholder variable names.

{{placeholder}} = Good

{{place_holder}} = Good

{{place holder}} = Bad

Enter your Google Doc template ID directly

If you can't find your template in the "Template Document" dropdown then try opening your document and selecting the long ID from the URL:

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Then, set the "Template Document" dropdown to "Custom" and paste in this ID:

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If all goes well, then the placeholder variable fields should appear in the zap editor.