I'd like to ask you about the Arduino ultrasonic sensor.

im using two type of ultrasonic sensor.... HC SR-04, US-015...

Measurement errors occur when the distance between ultrasonic sensors and acrylic is less than 5cm.

id like to hear in great detail the reasons for this situation...

Check the specifications of the ultrasonic sensors. There is a minimum distance that must be exceeded for reliable operation.

Utrasonics use a sequence of quick sound pulses not one quick blip.

So the distance traveled before the sequence completes is basis for the minimum range.