You should not need to change the data type to Whole Number of Currency (unless you are happy to lose accuracy or the column actually only contain values that will fit into integer or currency values). Decimal Number should work fine.
However it sounds like the column that you are having issues with may have been detected as a string column as strings can't be aggregated or the Summarize By setting has been changed to "Do Not Summarize". Double check the column type and if it's
string then changing it to Decimal Number should fix your issue. Or if the data type is correct then check that Summarize By is not set to "Do Not Summarize".
http://darren.gosbell.com - please mark correct answers