The sentence is missing a subject and the helping/auxillary verb. This has caused confusion because you have mistaken "I can" for the subject and verb. You have broken the sentence up incorrectly. The sentence breaks up into "[I am] Just screaming 'F'", "as loud as I can", "into the void".
The reader is left to assume that the subject is "I". The verb in this sentence is "am screaming".
"into the void" describes where the subject is directing their screaming. The subject is screaming at nothing in particular. They are screaming out into the universe.
"as loud as I can" is an adjective of "screaming". The subject is screaming as loud as they are physically able to scream.
Variations of "screaming into the void" are used in English to describe a feeling of extreme frustration. Imagine that you are so frustrated that you want to scream. The writer may even mean it literally!