Hi,
It is caused by the presence of dynamic contents with relationships in your Marketing Email.
For example, if there is a button which contains the calendar of a certain event, when you click the Send now button and send it to a contact directly, there will be an error message.

And the Test send will be frozen.

This type of marketing email can only be sent with customer journey. You can just preview dynamic text from contact information with the Test send feature.
Please refer to:
Deliver marketing emails quickly with send now (Dynamics 365 Marketing) | Microsoft Docs
In which, there is a requirement for emails that can use Send now:
Your message can include dynamic expressions, but you can't use relations. That means you can have just one hop (period) in your dynamic expressions, so for example, {{contact.name}} is supported but {{contact.contact_account_parentcustomerid.name}} isn't.