The answer to your question would be that the sentence error identified is the following one: DM (dangling modifier).
A dangling modifier is a type of misplaced modifier that modifies a word or phrase that is not clearly stated in the sentence. Dangling modifiers are also called "dangling participles". Participle phrases should sit right in front of the element they modify. If too much distance separates a modifier and its target, the modifier is misplaced. In this case, the present participle phrase, "riding his bike", modifies the NP "my brother". However, as the modifier is misplaced, it seems that the one riding the bike is the dog because it is the element most close to the present participle. Therefore, the sentence should be revised as follows: The dog was almost hit by my brother riding his bike. The sentence is still ambigous, so I think that the best option is to add a relative pronoun and turn the phrase into a relative clause (The dog was almost hit my brother who was riding his bike/when riding his bike)
