Can 26 x 1.5 inner tube fit 26 x 2.0 tire?
Yes it can but the slightly overstretching of the tube can increase your risks of puncturing.

Tubes marked with the same size as the tire are always slightly smaller than it and stretches upon inflation, so I always suggest going for an inner tube with its width size one step or level higher than the tire so that it won't have to stretch as far to conform to the tire's size, possibly making it more resistant to flats.