Alternating current cannot be measured by a D.C ammeter because:

Alternating current keeps changing its polarity periodically. In the positive half cycle, the alternating current has positive polarity, whereas in the negative half cycle, the alternating current has negative polarity with the same amount. Hence, the average value of the alternating current for one complete cycle is zero.

Let I = A sin(ωt)

Avergae value of current, Iav = ∫0T A sin(ωt) dt / T

Iav = A/T [-cos(ωt)]0T = A/T (cos(0) - cos(2πT/T)) = 0

Thus, alternating current cannot be measured by a D.C ammeter.