I literally am confused y the question and don't have a clue what to do here.

enum ParserError: Error {
    case emptyDictionary
    case invalidKey
}

struct Parser {
    var data: [String : String?]?

    func parse() throws {

        guard let data = data else {
            throw ParserError.emptyDictionary
        }

        guard data["somekey"] != nil else {
            throw ParserError.invalidKey


        }
    }
}

let data: [String : String?]? = ["someKey": nil]

do {
    let parser = Parser(data: data)
    try parser.parse()
} catch ParserError.emptyDictionary {
} catch ParserError.invalidKey {
}