If you want to create a business partner for each organisational unit you must specify this in customising.
After defining this setting, a BP will be created automatically when you create an OU.

You can find the menu in customizing:

In the first item you can define wether you want to create a BP for each organisation unit.

If you have already units in you organisational plan, you can use the second item in the menu to synchronise them. You can do this one by one or in bulk.

When you execute the query you can see all the units in your plan when a status(red/green). For the ones with a red light there is no BP defined. If you select the record and use the “start repair” button, a new business partner will be created for that ou.