When the movie Star Wars premiered in 1977, it was both surprising and a revolution in its special effects. what is the correct parallel structure

I'd write it this way:

When the movie Star Wars premiered in 1977, its special effects were both surprising and revolutionary.

Do you see it?

it's incorrect It states to try again without using were I have revised this twice and it's telling me it's incorrect

Without using "were"? No verb in the sentence??

In this sentence, the correct parallel structure can be achieved by using consistent verb forms. Here's the revised sentence with parallel structure:

"When the movie Star Wars premiered in 1977, it was both surprising and revolutionary in its special effects."

To ensure parallel structure, we changed "a revolution" to "revolutionary" to match the adjective form of "surprising." Now, both items in the list (surprising and revolutionary) are parallel in structure.