The fault code relates to low boost pressure, so without enough boost, less fuel will be injected, so less power.
So you have to find where the boost is getting lost, or why it is not producing the boost.
Start by checking the whole intake system. Start at the air intake, and follow the route through, to the air filter, then to the turbo, to and from the intercooler if fitted, then to the manifold. Every pipe, hose and connection needs to be carefully inspected for leaks, especially after the turbocharger. A trigger spray with warm water in might help, as it will blow at you from any leak area after the turbo.
If no problems found there, you need to investigate the turbo itself, and the wastegate. Does the turbo spin freely, and does the wastegate operate. It might be stuck open, so reducing pressure.
Also look at any exhaust gas recirculation system. These get dirty and stick open, reducing power.