At the beginning of Chapter 7, Tim and Pikachu will head to Ryme Wharf, a shipyard on the outskirts of Ryme City, after noticing it in one of the pictures found on Harry's Secret Locket camera. Apparently, this where the chemical known as R is begin both produced and stored. After arriving to Ryme Wharf, Tim and Pikachu will need to investigate by obtaining Testimony from the people working in the shipyard in order to Find out which of the three warehouses R can be found in.
After obtaining enough Eyewitness Testimony from the Pokemon and workers in the shipyard, you will determine (based on the shipping containers it holds) that the Warehouse you need to investigate is found in Area A of Ryme Wharf. After figuring this out, you will need to Find a way to infiltrate said warehouse. Thankfully, once you reach the warehouse in Area A, the next Fast Action Event will take place. Believe it or not, it will take you straight into the R Warehouse.