Are there any capitalization errors in Unless I agree to be her partner, Gabriella Montez will do the chemistry project on her own

No errors.

Explanation:

"Unless I agree to be her partner, Gabriella Montez will do the chemistry project on her own."

The 'U' in Unless is the first word of the sentence, so it must be capitalized.

The 'I' is capitalized because it is a pronoun (simple English language rule).

Gabriella Montez is a girl's first and last name, so they are appropriately capitalized as well.

chemistry doesn't need to be capitalized because it is simply a subject (with the exception of languages! For example, names of languages like French, English)