Ivancs,
Your question is too vague to answer. I have looked, and I don't see any FoxPro error message that reports "Invalid Input". This leads me to belive one of two things either: a) This is not a FoxPro message, in which case, I'm not sure there is a lot you can do about it, or b) This is an error message generated by code that someone has written. I assume in this case as well, that you are not the author of the application.
In the latter case, there are a number of places to look. "Invalid Input" leads me to believe that this is a field validation, and for whatever reason the data placed in the field is not valid data. Check your VALID clause on the field in this case, to see if there is code that does immedate checking of data enterd. This may also lead you to an error checking routine, which could be in a number of places. If it is capturing a real FoxPro error, you need to look in the ON ERROR routine, to see what "Invalid Input" translates to. (It has to be captured via the some function like ERROR(), or MESSAGE(). That may be caputred, and then replaced with custom error reporting. Usually this code would be kept in a MAIN, or PROCEDURE file, (as defined by SET PROCEDURE TO <procname>. The last place is, the code for this could be in the physical Screen Set (assuming this is a screen set), in the CLEANUP & PROCEDURES section. (Go to Screen, Layout, Code, Cleanup & Procedures).
If you can provide more information about OS, and the application, and what happens when you recieve this error message, I may be able to be more helpful.
Cheers,
-Scott Payton