Here's why:
* Subject-Verb Agreement: The sentence has a subject ("He") and a verb ("is"), but the verb is not in agreement with the subject. "Woeful" is an adjective, not a verb.
* Incomplete Thought: The sentence doesn't express a complete thought. It leaves the reader wondering *what* is woeful.
Here are some ways to correct the sentence:
* "He is woeful at singing." (This adds a prepositional phrase to clarify what he is woeful at.)
* "He is feeling woeful." (This changes the adjective "woeful" to a predicate adjective describing his state.)
* "He is a woeful singer." (This uses "woeful" as a noun modifier describing him.)