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How does the DM have an NPC guess where to attack me if my PC is invisible, since the DM always already knows? Unseen Attackers and Targets says:
When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see.
Does the DM have to pick a 5×5 square when playing on a grid? Should the DM roll the dice to choose?
Given that, I'd like to discuss the times when there is and isn't a known location of a creature. The RAW regarding unseen attackers and hidden attackers is fairly clear, although it is also somewhat counterintuitive to reality.
In this case, if the creature has Hidden successfully, then their location is unknown. They are unseen and have actively worked to not be heard or create anything that would give away their location. At this point, they'd just be guessing - and to guess, they'd have to know you were there in the first place.
In this case, it is up to the DM decide what location to pick if they know you're in the room, but not where. If it was a PC picking a random spot without knowledge of actual location, then that's fine.
But given that the DM knows your actual location, a random roll for potential spots is entirely reasonable and resolves the situation fairly.
Your quote box sums up the situation very well. The key phrase in there is a target that you can't see. There are other ways to detect the location of a target (hearing, smell, disturbance of the environment, etc.) that give away it's location to you. This allows you to know the specific location, but you attack at disadvantage. You are also not able to use any spells or abilities that require sight.
Commentary: Many people are frustrated by this lack of realism regarding invisible creatures having a known location. It is entirely within the purview of the DM to make the call that there are no external clues that would give away location based on their environment (effectively automatically Hidden without having taken the action to Hide), etc. However, the strict reading is that location known, but disadvantage given.
A simple, direct answer; by using senses other than their sight.
The location of an invisible, but not Hidden creature, should always be known. They're leaving marks, scuffs in the sand or make sounds. So an invisible but not hidden creature can be targeted with certainty, though usually with Disadvantage.
Being invisible also allows the creature to always qualify for hiding. If they are Hidden, then their location is not known and anyone would have to guess their location. They won't be targetable usually like this, though a Fireball may still catch them, or a very luck sword slash. The DM can simulate guessing by randomizing a target location in a room, for example (for a 4x6 room, throw a d4 and a d6 and use that coordinate as the target square).
This Dragon Talk episode has a section where Jeremy Crawford talks about Stealth in his Sage Advice column.
It mentions most of the above, and helps explain why even if people know where you are when invisible, you still have an great advantage over them.
Keep in mind that the reverse is also true. While invisible monsters are freaking scary, if they're not actively spending Actions for hiding, they just get advantage on you. You still hear them stomping towards you before they swing.
