Hi @Doraemon 

Thank you for your question in the Klaviyo Community 

It sounds like from your error message that the Template your are trying to send, has a blank Subject Line. To add one, if you are in the Template screen, is to select on the Pencil icon on the top left navigation. Here you should be able to enter the Subject. This will also become the Name of the Template when viewing all templates. 

You can also add one from the Template Listing Page, simple select on the 3 dots in the bottom right and then select ‘Edit Name’

I hope this is of assistance. 

Kylie

Thank you Kyle! I fixed and it worked!