Awkward syntax means your sentence is structured weirdly. Rearrange it so it flows smoothly and naturally.
Syntax is the structure of a sentence. It is also an area of linguistics (the science of language) which deals with where certain words go in a sentence and what their roles are – think of it like the anatomy & physiology of language.
If your teacher says ‘awkward syntax’ it means the structure of the sentence is a bit off or its oddly worded. Try rearranging the different parts so they run more smoothly and naturally.