Sometimes if an approval gate process is unassigned or deleted then the user can't choose another approval gate process in its place.
This is a known bug that will be resolved in future versions.
Workaround for 5.0: While in the approval gate window, move to another Application/Environment which has no assigned approval gate process. The "Add Approval Gates" button will appear. Now navigate back to the problematic Application/Environment and the button will remain onscreen. Click it to choose a new approval gate.
Workaround for 4.7: You will need to replace the old Process ID in the DB with a new Process ID (you will need to create and assign a new process to this environment).
1. Locate the Environment ID by running the following query on your DB: Select * from environments;
2. Locate the Process ID for the new process you want to assign as an approval gate step: Select * from process_in_env where ENVIRONMENT_ID=<%Environment_ID%>;
3. Replace the old process ID with the new process ID: UPDATE rc_step_a_env_def SET published_process_id=<%process_ID%> WHERE environment=<%Environment_ID%>;
The new approval gate step is now assigned in place of the old, missing one.