When the Esko verification tool doesn't find the spectrophotometer, it will return a "Failed to open device" message in the status field (bottom of the tool). This KB article gives an overview of possible solutions.
Make sure all other applications using / accessing the spectrophotometer are closed (e.g. Color Pilot, PressSync Pilot, i1 Profiler, ...).
Verify in the Device Manager (part of the Control Panel), if the spectrophotometer is correctly installed.
A correctly installed device has no exclamation mark symbol.
An X-Rite spectrophotometer is listed as follows:
Reinstall the i1 X-rite drivers from the IE DVD: \\Extra\Spectrophotometer_Drivers.