Initially,
So initially, the sphere will undergo uniformly accelerated motion and with decreasing angular velocity
the plank will move backwards with uniform acceleration
till the velocity of point of contact of the sphere on the plank wrt the plank is zero.
U should be able to proceed further
U cant conserve angular momentum abt point of contact because there is frictional work in this case and the point of contact does not have zero velocity. ie. Vcom≠0 initially