Every version of Excel has a maximum number of columns (in Excel2003, it was
around 256 columns, ending in Column "IV". In whatever version of Excel you
are working in, Excel thinks you have some content in the last column, and
cannot create/insert a new column without losing/deleting content that is in
that final column.
If you are intentionally using all columns, then you are out of luck -
you'll have to figure out a way to use fewer columns if you want to insert
new ones.
If you are not intentionally using all columns, then there are some
artifacts in some of the columns that you need to get rid of. Consider
highlighting the columns to the right of your "intended" data area, and
deleting them (not just clearing contents, delete the whole columns). That
should trigger Excel to recognize them as "empty", and then allow you to
insert additional columns.
HTH,
Keith
