Personalization Tokens in Lob Integration

Has anyone successfully integrated their Marketo account with Lob direct mailing?

I set up the webhook and built the letter I hope to send, but the Lob dashboard hasn't accepted the HTML letter. The error code says the Merge variables aren't designated, but they should be defined by the webhook, shouldn't they?

I want the letter to automatically pull from the contact data we have in Marketo for use in the letter. Can anyone tell me what I'm doing wrong?