Hi, I would like to know how to lock check box when a pick-list value is selected. the field i would like to lock is check box. I tried validation rule but it’s not working. Any help?

If you follow the Syntax provided by Akhil, you will need to wrap the TEXT function around your picklist field and in that case your validation rule will be as below

AND (
TEXT(pick_ist__c)="Sent",
ISCHANGED(check_check__c )
)

Please the insert field button to select the field API Names of the picklist and checkbox fields.