Last cell always IV4036

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I received a data worksheet from colleague, I tried to insert columns but
come back with the message that "Excel cannot shift nonblank cells off the
worksheet". I use CTRN-END and found the last cell is IV4036. I tried to
delete columns but still last cell is IV4036 and not able to insert more
column.

What is the problem and how to solve it?
Thanks.

 

[T. Valko]

See this on resetting the used range:

http://contextures.com/xlfaqApp.html#Unused