It's not missing, it's just that the transaction type isn't setup to allow transaction splitting.
You will need to check the company or transaction type settings
Safe Financials Toolbar
System Admin
Company details
Sales Main tab
Other sub tab
Invoice Splitting need to be set to INSTALL
If it's not set Right Click , Modify Record , update the details
Apply to save
You then need to check the transaction type set up as well, as the nominal updates set up may differ per company