Playground execution failed: /var/folders/8b/b2dxlhsd1s74pjswbjrr_7t40000gn/T/./lldb/377/playground449.swift:5:19: error: use of undeclared type 'Error' enum EnrollError: Error { ^~~~~ /var/folders/8b/b2dxlhsd1s74pjswbjrr_7t40000gn/T/./lldb/377/playground449.swift:5:6: error: type 'EnrollError' does not conform to protocol 'RawRepresentable' enum EnrollError: Error { ^ Swift.RawRepresentable:9:13: note: protocol requires nested type 'RawValue' typealias RawValue ^ /var/folders/8b/b2dxlhsd1s74pjswbjrr_7t40000gn/T/./lldb/377/playground449.swift:16:31: error: thrown expression type 'EnrollError' does not conform to 'ErrorType' throw EnrollError.notAvailable ~~~~~~~~~~~~^~~~~~~~~~~~ /var/folders/8b/b2dxlhsd1s74pjswbjrr_7t40000gn/T/./lldb/377/playground449.swift:19:31: error: thrown expression type 'EnrollError' does not conform to 'ErrorType' throw EnrollError.notQualified ~~~~~~~~~~~~^~~~~~~~~~~~ /var/folders/8b/b2dxlhsd1s74pjswbjrr_7t40000gn/T/./lldb/377/playground449.swift:22:31: error: thrown expression type 'EnrollError' does not conform to 'ErrorType' throw EnrollError.alreadyEnrolled

抱歉，可能是由于Swift版本的问题导致代码无法运行。以下是更新后的代码，希望能够解决问题：

enum EnrollError: Error {
    case notAvailable
    case notQualified
    case alreadyEnrolled
}

class VendingCourse {
    var availableSlots = 3
    var isQualified = false
    var isEnrolled = false
    
    func enroll() throws {
        guard availableSlots > 0 else {
            throw EnrollError.notAvailable
        }
        
        guard isQualified else {
            throw EnrollError.notQualified
        }
        
        guard !isEnrolled else {
            throw EnrollError.alreadyEnrolled
        }
        
        availableSlots -= 1
        isEnrolled = true
        
        print("Enrollment successful!")
    }
}

let course = VendingCourse()
course.isQualified = true

do {
    try course.enroll()
} catch EnrollError.notAvailable {
    print("No available slots.")
} catch EnrollError.notQualified {
    print("Not qualified to enroll.")
} catch EnrollError.alreadyEnrolled {
    print("Already enrolled.")
} catch {
    print("An unknown error occurred.")
}

如果您还遇到问题，可以告诉我具体的错误信息，我会尽力帮助您解决问题。