Answer: The subject and verb do not agree.
The subject and verb in a sentence must agree with one another in number i.e. singular or plural. It means that if the subject is singular then the verb must also be singular. If the subject is plural then the verb must also be plural.
The given sentence is in the present tense so the noun and verb must form singular/plural in opposite ways: by adding 's' to the nouns and removing 's' from the verb to form singular.
The correct sentence would be: Brad and Alicia say they are available to babysit on Friday night.