I'm Lucy, and I'd be happy to answer your questions today.
A person who has an easement can't build on it in a manner that prohibits the holder of the easement from using it. However, if you have a easement for drainage only, and you're not interfering with drainage by having your fence where it is, the general easement statutes don't say anything about that. You may want to ask them WHICH Florida law prohibits having a fence on an easement. People love to say things like "State law says...." without ever verifying that such a statute exists. If they're saying that the ACTUAL easement prohibits anything being built on it, check the language of the easement. That's a little different.
The law of adverse possession says that a person who occupies the property of another, exclusively and continuously, for the statutory period, gains ownership of the land. But that doesn't really apply to your situation where it's undisputed that you own the land and they're talking about an easement. It is possible to REGAIN land beneath an easement by preventing the easement holder from using the property for 10 years - but again, if the purpose is drainage, and it's still being used for that purpose, then building the fence wouldn't have any affect.
See if they'll clarify what law they think you're breaking. If it's an issue with the CC&Rs, that's a little different. But what you should know is that, if you build where the HOA has told you not to, they can impose fines on you every single day until you go to court or until you take the fence down. So, it's when a homeowner has this type of dispute with an HOA, it often helps to have a local attorney reach out to them before acting, so you can hopefully avoid a lot of trouble down the road.