If you can't message students in Handshake, this is most likely related to your recruiting activity status at the school you're trying to connect with.
Messaging students at partner schools
To proactively message a student at a partner school, Basic and Pro employers must have at least one approved or closed job posting at that student's current or former school.
For step-by-step instructions, refer to How to Send a Student a Message.
Messaging students in the Handshake Open Network
To connect with students and alumni in the Handshake Open Network, Basic and Pro employers must have at least one approved or closed job posting at any partner school on Handshake.
For more information, refer to How to Connect with the Handshake Open Network.
Exceptions for Essentials and Enterprise plans
Essentials and Enterprise customers can message students without creating a job posting.