You have not specified if it is the warlock or the target who is subject to the invisibility spell.
In either case, invisibility does not provide cover.
In addition, being invisible does not make you hidden. To be hidden, you must have taken the Hide action and subsequently not done something to lose hidden status. Unless you are hidden, people know where you are. See What advantages does hiding have? One thing that would cause you to loose hidden status would be to be at the end of "a sustained arc of lightning", like from, hmmm ... Witch Bolt
If the warlock is subject to an Invisibility spell then casting Witch Bolt or any other spell ends the Invisibility spell. Being an unseen attacker would grant the warlock advantage on the ranged spell attack.
If the warlock has already cast the Witch Bolt and then is the target of an Invisibility spell they can continue to concentrate on the Witch Bolt and do damage each round without ending the Invisibility since they are neither casting a spell or attacking. Of course, they cannot Hide without ending the Witch Bolt as that would take their action. If they can Hide as a bonus action or have more than one action per turn, they can Hide but would immediately lose hidden status due to being at the end of "a sustained arc of lightning"; just because you can doesn't mean you should.
An invisible target gives you disadvantage on the ranged spell attack. In addition, if the target is also Hidden, you would need to guess its location; if you guess wrong you will automatically miss. If you are playing on a grid this would mean indicating the square your think the target is in; if not playing on a grid it would mean indicating in some other way.
On a hit, the creature would remain invisible but would not be hidden nor be able to Hide while subject to the spell.