Parallelism or parallel structuring in a sentence is when there is the need for the sentence to have a common or same grammatical structure or construction in the related sentences. This parallelism makes it easier for the text to be understood and allows easy readability.
In the given sentence "<em>After school we planned to do homework and swimming</em>", there is lack of parallelism. The sentence makes it a bit awkward by placing homework and swimming right after "we planned to do". Rather, if another word is added, like "and go" after "homework", then the whole sentence will be more improved.
So, the error is the lack of parallelism or parallel structure in the sentence.
