We often use "one" to substitute for "a person" or "he or she". That's true for this sentence, too.
"One cannot communicate" is grammatically correct alone as a simple sentence. However, some more information would help it make sense. It's probably part of a larger sentence, like:
When asleep, one cannot communicate.
One cannot communicate complex ideas to babies.
In other words, When a person is asleep, he or she cannot communicate. Or, for the second sentence, A person cannot communicate complex ideas to babies.
Sometimes, people use the word "you" for the same purpose. Both "you" and "one" can be used as what we call generic pronouns. In some situations, you can use either (or--"one can use either"! See?). However, "one" is more formal than "you", so I recommend using "one" in things like school papers and "you" in casual conversation.
If you understand how "one" is used, maybe you want to know the meaning of "communicate". I recommend that you use an English Learner's Dictionary to help find definitions in simple English. For example, Merriam-Webster's Learner's Dictionary provides 3 main entries for "communicate":
1 : to give information about (something) to someone by speaking, writing, moving your hands, etc. [+ obj]
2 : to get someone to understand your thoughts or feelings [no obj]
3 [+ obj] medical : to pass (a disease) from one person or animal to another
In all 3, the main idea is exchange, or sharing.
If we put all that together, we can understand that depending on the context, another way to say "one cannot communicate" might be "a person cannot get someone to understand his or her thoughts or feelings" or "you cannot give information to someone else."