Steps to reproduce issue
1. Create a new Data set
2. Create a variable in Data Set and assign it to some value
3. Create a JR Table and assign the created data set to the table.
4. Right click a column in Table Footer from Left Panel and add a cell.
5. Now Just click the column and on the right you will see "Column Print When" in the properties section, here add an expression to print only when the created variable to the Data Set is not empty.
6. Compile the report, error is thrown as "Variable Not found", But the variable is available in Data set. The same this works when a text field is added to the cell and an expression to print only when the created Data Set variable is not null.