This page lists the error messages the GEDCOM Estimator may give you, and explains a bit more than what is possible in the message box itself.

"The specified GEDCOM file was not found"
The input GEDCOM file you specified does not exist, or the program is unable to read it. If you select a file in the file dialog rather than typing in a name, it should be OK.

"Could not write subset file"
As mentioned in "How to do the estimation", the program will store one or more subset GEDCOM files if negative year ranges were encountered during estimation. This message will arise if the program for some reason was unable to store one of the subset files. Likely reasons are that the device is full or write protected.

"Could not write the output file"
This means that the program could not write to the output GEDCOM file you specified. Again likely reasons are that the device is full or write protected, or that there already is a read-only file with the name you specified.

"Illegal number value! The values can only be whole numbers."
This message can arise when you try to set options in the options window. The values for the otions must be integers, i.e. whole numbers, and nothing else. Check that you haven't typed e.g. a letter by mistake and try again.