I could not understand this statement "System will not allow to reverse a journal in GL from a frozen source".
I just tried this:
1. Created AP invoice
2. Validated and accounted / posted via SLA
3. Checked in GL - it was posted.
4. Then it allowed me to reverse that journal batch in GL
Per my understanding - system should not have allowed to reverse this journal as the source transaction is frozen and from the process/compliance standpoint - "cancel invoice" must have been utilized to reverse the journal entries in GL. Please clarify what this frozen source means? Thanks

Go to the Source screen through your GL responsibility.
See if Payables source is frozen?
You will not be able to reverse posted journals from frozen sources.
Not only that you cannot delete journals from frozen sources as well.
The GL Source screen will tell you if the source is frozen or not.
This topic is covered in the videos.
Hope that helps.