You need to obtain the Key Item Silph Scope in order to be able to see the ghost
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Get a Slith Scope and climb the towe. Beware: the ghost Pokemon that you visually see transform cannot be caught with a Pokeball
You need the Silph Scope to see it, then defeat it.
first at sprout tower at violet city. next is burnt tower at ecruteak city.
Because Ghost Pokemon can't see in the dark.
you need to find the silph scope this will reveal the ghost to be a marowak you can then defeat it