error T0021: Cannot mark a system description as optional(!).

Cause

This error occurs when a system description is marked as optional.

Solution

Do not mark the system descriptions as optional.

Example

[!System: Variables]

In the above example, the system description is marked as optional. Hence the message error T0021: Cannot mark a system description as optional(!).

The code is corrected by removing the symbol !.

[System:Variables]