WatchKit Can not set the Label title

Question

I have tried to set the WKInterfaceLabel title programmatically but, It is not changing.

WKInterfaceLabel *titleLabel;

Couldn't set the title by using this,

[titleLabel setText:str];

Also tried Attributed string too,

NSMutableAttributedString *attString = [[NSMutableAttributedString alloc] initWithString:str];
[titleLabel setAttributedTitle:attString];