Diagnosis
There are two possible reasons for this:
Decision tree has not been released.
Decision tree has not been assigned.

Procedure
In the Customizing activity, Define Tax EventDetermination (Cross-Application Components -> Transaction TaxEngine -> Tax Maintenance Settings -> Tax Determination and Calculation-> Define Tax Event Determination), call up the decision tree andchoose Decision Tree -> Release.
Note: Only a released decision tree can be used for tax eventdetermination. Even if the Valid-From and Valid-To datesare in the past, only a released decision tree can be used.
Assign the decision tree as described in in the sub-activity AssignDecision Tree contained in Assign Tax EventDetermination and Calculation Rules (Cross-ApplicationComponents -> Transaction Tax Engine -> Tax Maintenance Settings -> TaxDetermination and Calculation -> Assign Tax Event Determination andCalculation Rules)