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.

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{{placeholder}} = Good

{{place_holder}} = Good

{{place holder}} = Bad

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Enter your Google Doc template ID directly

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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.