why is osmotic pressure is positive and osmotic potential is negative??

Osmotic pressure is a hydrostatic pressure exerted to the solution to prevent the flow of water through the semi permeable membrane. This pressure is generally created by the solute present in the solution. As a solution be it hypertonic or hypotonic will always have solute in it, the osmotic pressure will always be there may be more or less. Thus, the osmotic pressure of a solution will always be positive.

Osmotic potential is the potential of water molecules to move between solutions of different concentration through semi permeable membrane. The osmotic potential of pure water is taken as zero as there is no solute present in it. As a solution will always have some solute, the amout of water will be lesser than pure water. Thus the osmotic potential of a solution will always be negative (value less than zero).