This results in a contradiction
A observes B's clock to run slow, B observes A's clock to run slow. It seems contradictory but it isn't.
It would only be a contradiction if simultaneity were absolute but it isn't, simultaneity is relative.
For A to observe the rate of a clock stationary in B's frame, A must use two spatially separated and synchronized clocks stationary in A's frame.
Similarly, for B to observe the rate of a clock stationary in A's frame, B must use two spatially separated and synchronized clocks stationary in B's frame.
Why? To record the start and end time on B's clock, A must have a clock co-located with B's clock at the start and at the end. But B's clock is moving in A's frame; B's clock location has changed between the start and end times. Thus, A requires two spatially separated clocks and these must be synchronized according to A for the measurement to be valid.
And there's the resolution of the contradiction: A observes B's spatially separated clocks to be unsynchronized and vica versa.
This is so well known and so well understood and this question has been asked so many times here and elsewhere, I suspect your question will be closed in short order.
For further reading, see “Moving Clocks Run Slow” plus “Moving Clocks Lose Synchronization” plus “Length Contraction” leads to consistency!