One of the subVIs on the block diagram is currently running, and it has not been compiled for reentrant execution. Unless a subVI is compiled as a reentrant subVI, it might not be safe to call that VI again when it is already running. Doing so can produce incorrect results or corrupt the data in both calls to the subVI. You need to wait for the subVI to finish executing before you can run this VI.
To avoid this error, right-click the subVI icon in the upper-right corner of the front panel or block diagram and select VI Properties from the shortcut menu to display the VI Properties dialog box. Select Execution from the top pull-down menu of the VI Properties dialog box and place a checkmark in the Reentrant Execution checkbox. The next time you run the VI, LabVIEW recompiles the subVI for reentrant execution.