This response assumes that ValMatch is version 1.90 or higher.
Every sample and standard measurement has a sensor type and serial number associated with it. The error message is occurring because you are trying to match a sample that does not have the sensor serial number loaded (does not have to be the Active Sensor, just in the list of Available Sensors) that was used to create it.
My best guess is you may have copied the database files from version 1.80 to version 1.90 but did not install all of the sensors (including demo sensors like CQDEMO) that had been used in 1.80 to create some of the samples or standards that you are trying to correct.
To determine which sensor has not been installed, go to Data/Recall Sample and recall all of the samples. If the sensor is not installed for some samples, a message will appear telling you which one. Install that sensor manually and this message should disappear. If it happens on a standard, Data/Recall/Standard to tell you which sensor is not installed.
Just install that sensor as an Available Sensor (does not have to be the Active Sensor) and this should take care of the issue.