How to validate and display error message without making the screen disabled

I am using Enhancement AIST0002. Inside the exit exit_saplaist_003 i am coding  and validating Fields Lease Length and Notice Date and Lease Start Date.

But there should be an error message displayed when Notice Date and Lease Start Date difference is more than 99 years.

I am throwing a message for this. But every time the message is thrown the whole screen is disabled which shouldn't be the case. The screen field should be active.

Please guide me on this.