Hi Cindy,
There can be a few causes for this warning.
-The name of the shapefile (The shapefile name may begin with a number).
-The name of the table.
-The file name or its path contains space characters, especially with dbf files.
-A field name in one or both files contains an invalid character, such as a slash, space character, parenthesis, and so forth.
Try removing invalid characters, or rename the file or fields causing the problem. Rename the file/field to omit the invalid character(s). After the file/field has been renamed, the table join must be reestablished.
Hope this helps.