Can we measure amplitude of any frequency signal using digital multimeter?

ANSWER

No. A DMM has a rated frequency.

Some DMMs are "true RMS" meters, which either means (a) they digitally sample the waveform at some relatively high frequency and perform the necessary math internally, or (b) they accomplish the same thing using an analog circuit. In either case, there is a maximum frequency they can accurately measure, because of the sample rate or because of the filter frequencies in the analog circuit. Typical meters can range from 1 kHz up to around 20 kHz.

Other DMMs do not make true RMS measurements. These make the assumption that the waveform is sinusoidal, so they merely rectify and filter the signal and then apply a scaling factor. The filters limit the applicable frequencies. These are often either very inexpensive meters, or are built specifically for AC power line measurements.

At some point, it's better to perform the measurement using an oscilloscope (with a high enough bandwidth).

It's not always easy to find the frequency rating for a digital multimeter. Not all manufacturers include it in the specifications.