Cannot resolve the error Collection element of type 'void*' is not an Objective-Object (old title; Expected'; 'at end of declaration list)

The underlying problem is

NSArray*_lines=@[@"A", @"B", @"C", nil ];

The last semicolon of is missing.However, the location of the declaration and initialization of the variables is not good, so the code you want to write is probably as follows:

#import "ThirdViewController.h"

@ interfaceThirdViewController()

@end

constintChangeCheckCell=1;
NSMutableArray* checkboxArray;
NSArray*_lines=@[@"A", @"B", @"C", nil ];　　　　　　

@implementation ThirdViewController


- (void) viewDidLoad{
    superviewDidLoad;

You cannot declare variables that are not inside a function between @implementation and @end.
ThirdViewController If you want to declare a variable that is visible only in ThirdViewController.m that is not a member of the class, you should write it outside @implementation.

Also, if checkboxArray and _lines are members of the class, the variable declaration should be done between @interface and @end in ThirdViewController.h to initialize the variable with viewDidLoad>.

Corrected questions

The {} after the @implementationThirdViewController is unnecessary.