Synthesia 'Convert video to template' error: API variables only work in templates

Do not try to generate the variable-laden video directly. Convert it to a template in the editor, keep the variable names stable, and pass the per-recipient values on each API create call along with a callbackId so you can track who each video belongs to.

Context: If Synthesia refuses to generate with 'Convert video to template', the video contains API variables (personalized placeholders). Per the Synthesia knowledge base, API variables are only compatible with templates, so the video must be converted to a template before generation. This is the designed flow for personalized videos at scale: build the template once with variable slots, then generate per recipient with variable values.