The expression you entered contains invalid syntax

Hello good friends!
My post here is about my problem this morning while using Microsoft Office Access. I pasted the screen shot of the error for better understanding on my issue.
Microsoft Office Access
The expression you entered contains invalid syntax.
You may have entered an operand without an operator.
As I am quite new in using Microsoft Office Access, this is my first time to encounter this error message. I asked some help from my buddies, but they don't have any idea about this error message. Please share your thoughts on this error message that will guide me in solving this problem. All your ideas will be highly appreciated. Thanks a lot for reading my post.
