I think what happened here is that there was already a suggested edit pending on that post when you reviewed it. The timeline looks like (all UTC):
- 18:57 - Suggested editSuggested edit is proposed.
- 19:09 - You review the post and try to edit it
- 19:12 - Suggested edit is reviewed and processed
During that window between 18:57 and 19:12, you would be unable to suggest an edit on that post since there is already one awaiting review. Thus, when you tried to make some corrections to the post in the review queue, it returned an error.
Therefore, I'd say the main problem here is that the text of the error message is very poor. If it had said something more like:
You may not currently suggest an edit to this post because one is already pending.
...then it would have more accurately captured the situation.