The error is shown by the following scenario.

Login in as Admin User to the AWI and create a simple, restricted user. Assign no User Group and only give the privileges shown in the screenshot below:
 

Next, create a workflow with one job for this user. That means, the name of this workflow must start with WRZKLB
Start the workflow with Execute once - Wait for manual release:

Log in as the newly created, restricted user to AWI and open the Process Monitoring. There you see the job in Waiting for manual release:
 
Open the Monitor. There, Modify can be clicked, i.e. it is editable: Right click on the job and select Open Monitor:
 
The JCL is opened. The Modify button is greyed out (deactivated) and the JCL is write protected and not editable:
 
For comparison, open the Java UI (12.0.5) with the restricted user and do the same procedure. I.e. select Monitor - Open generated Job. Here the JCL is not write protected and can be edited and saved:
 
I.e, using the same user rights (see first screenshot above), the behavior is different in AWI and Java UI.
In Java UI (V12 and older), editing the generated JCL was and is possible with these rights (see first screenshot above). In V12 AWI it is not possible anymore.

Release: AUTWAB99000-12.2-Automic Workload Automation-Base Edition
Component:

The issue is still under investigation by our development

Workaround:
Give the users the right W- Write for these objects or use the Java UI (use of Java UI is restricted in V12.1 and newer, please see this community post)