Administrator users enable approvals by going to the Administration side-bar menu, then clicking on "System settings":
Scroll down to the "Expense compliance" section and check the "Approvals enabled" box.
This will expand a section below that can be used to make approvals optional. This is a special-case workflow that is only very rarely needed and we strongly recommend that "Approval required" is always left enabled.
If you think you might want to turn this off, it's worth checking in with RIPA Support first just to make sure that this is the correct solution to the business workflow problem you are facing.
After this, you will need to configure your teams.