"A plate is not food."
Translation:צלחת זה לא אוכל.
It's either צלחת היא לא אוכל which is a bit stilted for fluent speech or צלחת זה לא אוכל. In the first case, היא is a copula which agrees with צלחת. In the second case, זה is a copula/demonstrative that agrees with אוכל, in a similar construction of the English sentence "This is he". You could think of it like צלחת, זה לא אוכל A plate - that isn't food, if that helps you with the agreement.
Hope that helps
OK, I guess I need a primer on grammar. I saw זה was an option and היא also an option. Not sure why you can use either, or if it's just a flexible choice. The correction to my mistake (היא) wasn't even an option in the selectable words but זה was and I'm not even sure how I would have been aware of these varying options. Thanks for the help.
A lot of learners have problems understanding the copula. It's normal. Generally, copula must agree with the gender of the first noun, which would be היא here. However, זה is also widely used, regardless of the gender.
Try remembering the specific examples, in the beginning, write them down and review them and after some time you'll get a better feel for which copula to use when.