Error message: No error message. You'll see text missing from the Measures section in the PivotTable where it should read "Activity".
You may also have luck following the steps in the adjacent help site article here.
"Activity" Text Missing from PivotTable - Measures Section
Description: This happens from an unintended change to the Value Field settings, which is accessed by right-clicking on "Activity".
By default, "Custom Name" = "Source Name". Since "Custom Name" is auto-selected, if one inadvertently presses any key on the keyboard it will replace "Custom Name" with the value from the keystroke. In the example above and below, the key that was pressed was the Space Bar.
Solution: Open the Value Field Settings for Activity and replace empty value or keystroke with the work "Activity" to restore the name.