The import tool can only read Olympus/Innov-X data files which have been captured in either Soil mode or Mining mode.  This is generally designated by either an “S” for soil or “M” for mining.  This error message means that there is an entry in the Mode column which is not one of these (and may be even a null/blank entry) and this must be fixed or removed before the data can be imported into ioGAS.  Other modes, such as Two Beam Mining, etc. should still import OK provided that all the entries in the Mode are the same.