You cannot obtain the measured value with the FETCH query alone unless you have previously sent a MEAS or READ query.
The FETCH query retrieves the value from the last MEAS or READ query, so using a FETCH query by itself will not returns the expected value.
Here is the command example how to use the FETCH query effectively when retrieving multiple parameters measured at the same moment in time:
MEAS:VOLT:AC?
FETCH:CURR:AC?
FETCH:POWER:AC?
The above commands will acquire the voltage, current, and power measurements taken at the time the MEAS command is sent, ensuring simultaneous AC power measurements.