Dogs urine ruined my air conditioner

You say the unit's being rented - if you're the tenant, you need to complain to your landlord so he or she can take this up with the board. If you're the owner, maintenance IS your responsibility, so it may be impractical for you to check, but such is life as a homeowner, whether you rent the place out or not.

Since air conditioners are the owner's responsibility, I don't see why your board wouldn't allow this unless the placement of the fence would interfere with nearby air conditioners. You might consider attending the next meeting talk to the board and perhaps have the air conditioner repairman prepare a written statement for them to review. Since the discussion might get lengthy, you might want to ask the president if you could be placed on the agenda - you'll have a little more time than bringing it up in a resident's forum. Better yet, have the repairman attend the meeting with you so the board can ask questions.

I must admit, I was a wee skeptical of such damage to an air conditioner unit until I Googled it - sure enough, this is a thing - https://georgebrazilhvac.com/blog/why-you-should-keep-your-dogs-away-from-your-a-c
You might also talk to a few of your neighbors to see if they have the same problem - the more responsible dog owners might even have some suggestions that don't involve a fence (at first blush, I can't think of any, besides using animal repellant, but that will require reapplication after it rains)

I am the owner, so it is impractical to ask tenants to keep an eye on my air conditioner, as it not their responsibilities.
I talked to some neighbors, one of them also suggested me to add a fence. I'll talk to the board. Thank you for your suggestions.

You could try a dog repellent. Either the chemical scent based ones, or the ultrasonic sound based ones.

I had a neighbor with a lab and no fence a few years ago. They would let it out to do its business and it liked to do its business in our yard. The the mower wheels would track it into the garage. I got a motion activated ultrasonic dog repeller and it worked reasonably well. The dog would still run into our yard when playing fetch (which I didn't mind), but when he wanted a relaxing spot to squat for a minute, he would chose another place. That was exactly what I was aiming for. I would point out, that even though they say people cannot hear them, people under 50 likely have good enough hearing to faintly notice it.

Tim,

Dog urine (the dog will pee on the same spot every time) WILL eat aluminum.

The vast majority of 'heat transfer fins' are aluminum.

Took the time to look on the internet.

Here are some suggestions:

Plant bushes. This may keep your dog's spray from reaching the condenser, even if he chooses to mark his territory near your AC unit.

Build a fence.

Use odor sprays (one can be made on your own, vinegar, water and orange essential oil).

One other suggestion was to spray the compressor fins with a mist of WD-40.

If you can't put a fence or shrubs around the compressor, and can't (or don't want to) pay for stainless steel unit, I think that a coating of clear flex seal spray would provide the longest protection with (I think) minimal loss in heat transfer.

Put chicken wire around air conditioner.
Attach electric fence unit to chicken wire.

Dog will quickly learn not to pee on it without injury.

Thank you all for your suggestions!
I talked to an experienced investment agent who also has several rental properties. He said, owner is responsible for the maintenance of air conditioner, like normal wear, but NOT responsible for the damage caused by pets urine. The AC is put on common area, HOA is responsible for any damage, and that’s why they need to buy insurance. I am going to talk with HOA. Hopefully their insurance will cover the cost to fix AC.