Ash here from the Shopify team. Welcome to the Shopify Community!
I understand that you've used the Klaviyo application to create a custom discount, but are having some difficulty getting it to work at checkout. I'm happy to assist with this! I'll need to get some more information from you in order to assist to the fullest, so could you please reply back to this thread with the following information:
I look forward to hearing back and getting this sorted out!