N0nick , suggest to ask similar questions in the dedicated button-card thread.
Besides, “Include template” is a rather confusing title. Should have mentioned “button-card” there - which is a CUSTOM component not commonly used.
And try using this:
button_card_templates: !include_dir_merge_named templates/button-card/
w/o leading “/”.
And the “button_card_templates.yaml” (which you placed in “templates/button-card/”’ folder) must NOT contain the “button_card_templates” line.
Also, the whole idea of “!include_dir_merge_named” is keeping MANY files in the “button-card” folder - like “one file per one template” or “one file for many related templates”.