You should read a textbook on basic electronics measurements.
In short, there are two problems: the frequency response of your operational amplifier, and second, loading the signal source by the circuit.
To reliable measure AC voltage, you should do it on a known source impedance, and your voltmeter should be matched to it. At higher frequencies, "a detector" becomes to have stray impedances added which should be taken into account.
Another approach was adopted by R&S many years ago: they designed an AC and RF voltmeter with a high-impedance detector which was designed to have a flat frequency response up to > 200 MHz.
At higher frequencies, voltage is no more a reliable quantity, so power sensors are preferred.