Can I say "Lubię ciebie"?
Not really, unless you want to emphasize 'ciebie' (e.g. "Lubię ciebie a nie jego" - "I like YOU and not HIM").
Most of the time you'd say "Lubię cię".
When do I use "cebie" and when "was"?
"ciebie" is an empasized variant of "cię", so it would rather work in a sentence like "I like YOU, not him!".
"Lubię cię" = I like you, Sven.
"Lubię was" = I like you, Sven and Matilda.
So is 'was' used if you were going to say "I like you" and 'you' is more than one person but only one unit? Like a married couple?
Lubię cię = I like you
Lubię was = I like you as a couple
Yes, but it doesn't necessarily need to be one unit. You're just addressing multiple people.
It said another accepted phrase was 'Lubię was' .... was was not even in the selection of words to choose from.
Why not "Lubię cię." ?
It's one of the starred answers.