A sign is a sign, but not just anyone can issue a fine. The HOA doesn't have the authority to write fines for violations - at best it can demand compensation for damages through court if the lawn really was damaged. But the municipal inspection authority or the district administration - yes, they can draw up a report and impose a fine. Though that only applies to general maintenance standards on common property.
Now to the main point - whether the requirement is even legal. The Housing Code says that common property in a building (including the adjacent territory) must be maintained in proper condition. And if the owners voted at a meeting to ban parking on the lawn, that decision can be formalized in the territory usage rules. But the HOA itself can't issue fines - that's the job of the inspecting authorities. The HOA can only file a complaint with the administration.
If there's a sign but you didn't vote for the ban at the owners' meeting, it's worth checking whether there even was a meeting and what decisions were made. If there was no decision - the sign is just hanging there with no legal force. Request the meeting minutes and documents on this issue from the HOA, and while you're at it, find out who exactly is authorized to issue these fines.