Electric force produced by sheet will accelerate the block towards the sheet producing an acceleration.
Acceleration will be uniform because electric field E due to the sheet is uniform
As initially the block is at rest and acceleration is constant, from second equation of motion, time taken by the block to reach the wall
As collision with the wall is perfectly elastic, the block will rebound with same speed and as now its motion is opposite to the acceleration, it will come to rest after travelling same distance L in same time t. After stopping it will be again accelerated towards the wall and so the block will execute oscillatory motion with ‘span’ L and time period.
However, as the restoring force F=qE is constant and not proportional to displacement x, the motion is not simple harmonic.
