Services in the Behavior Tree sometimes don't work when the project is first loaded. The Behavior Tree will print a "missing service node" warning and the BT will remain inactive.
This was reported and tested in 4.20.3 (CL-4369336). This was reproduced in 4.19.2 (CL-4033788) and Main 4.21 (CL-4387437)
Results: LogBehaviorTree: Warning: MyBehaviorTree has missing service node! (parent: MyTask)
Expected: No warnings are thrown and the Behavior Tree executes
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-64388 in the post.