After having created a new purchasing organization, the next step is to assign it to both a company code, and a plant, in order to be able to use it within the MM module. Otherwise, some issues may be faced while creating Purchase orders for example.
In the change view, find the Purchasing organization to which a company code should be assigned, enter it, and save
A customizing request will be necessary and a prompt with ask for it
Then, back in SPRO, navigate to Enterprise Structure > Assignment > Material Management > Assign standard purchasing organization to plant
Find the plant to which the purchasing organization should be assigned
Enter the purchasing organization, and save
Again, a prompt will request a customizing request
More information on the corresponding SAP thread