If you have launched a sequence and are seeing some contacts fall into the Issues tab under your Enrollment tab, in some cases this simply means the system is doing its job based on your settings!
In the case of the message, "Your email validation settings are preventing this contact from starting the sequence," this simply means that the contact's email validation status does not fall within what you have set inside the sequence settings, so the system is working as expected based on your settings and preventing the system from sending to that particular contact.
If you click on the contact to open up the Contact Details page, you will see the contact's email validation status when you hover over the checkmark to the right of their name.
If you click on the Sequence Settings, you will see what you have your email validation settings set to. For any contact who has a status other than what you have your settings set to, if the toggle is turned on to validate emails before sending then it will prevent sending and those contacts will fall into the Issues tab as shown above when the system attempts to send them an email from the sequence.