What is the difference between the following two sentences
You must not read such trash
You must not read such a trash
Or are they both the same?
You must not read such trash. Trash, meaning "something which is waste or of no value" is a non-count noun, so you would not use an article such as "a" before it.
trash
noun [ Uncountable ]
anything that is worthless and of low quality; waste: