Example: If you have the token for first name written like this {{lead.FirstName:default=Hello}}, when you try to approve the email Marketo will let you know that the token value is incorrect. The correct token value is {{lead.First Name:default=Hello}}. For this example if you change the token value to {{lead.First Name:default=Hello}}, Marketo will allow you to approve the email and it will allow you to do a send sample.
Example: If you have a token in the Reply-To like this one, {{lead.Email Address:default=edit me}}, then the Send Sample using the default will have "edit me" in the Reply-To, which is not an email address. Marketo cannot send an email without a Reply-To email address, so the Send Sample will fail.
If any of the above does not apply, run the following tests and provide the information to Marketo Support