For our better research, please provide more information about this case:
Do templates refer to templates saved locally or templates from the My Templates app?
How did you create and save the template?
According to the test, if you need to create a large email template, it is more recommended to create a local email template in the Outlook desktop client. If you are using the My Templates app, there will be a limit, and this error will occur when the total size exceeds 32kb. For more information, please refer to this article: Template is too large to save or template couldn't be saved error in the My Templates app in Outlook Web App .
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