Thanks for the reply!
Yes, I tried running the Qubes Policy Editor (with sudo qubes-policy-editor in dom0).
It did open, but instead of the graphical window, it only showed a black text screen in the terminal (looked like nano).
So I couldn’t really see where the error is highlighted.
Do you mean that the Policy Editor GUI should show the exact line with the syntax problem?
If yes, maybe it’s not running correctly in my system.
Is there another way to make it display the specific error location (or check the syntax manually)?