Diagram Utility Trace intermittently does not allow "Run To Line" command on first loading of an action diagram with message:
The operation is unavailable on the current selection. Please place cursor on valid line to run to.
The root cause appears related to the heap allocation for the DTU.
Edit the DiagramTrace.ini file found in the Gen\dt directory e.g. %GEN86%\Gen\dt\DiagramTrace.ini.
Increase the maximum (-Xmx) java heap from 256M to 1024M (1GB) and also make min and max heap the same (for improved efficiency at runtime)
If desktop RAM is limited and there is a concern about impact to overall performance can first increase to 512M to see if that will be enough to resolve the problem.
If running DTU against a remote application the potential of any impact to system performance by increasing DTU heap should only be on the local machine/desktop running the DTU and not on the remote server hosting the application.