After signing up at https://app.tabnine.com/signup,
You should have received the activation email ('Verify your email for Tabnine') from firstname.lastname@example.org.
Are you sure it did not get to your spam folder? Please check the junk/spam.
In many cases, an organization (university/company) holds the message back and puts it in quarantine.
Would you please check with your IT department if our address may be blocked or marked as spam?