This works for me:
let imageAttachment = NSTextAttachment()
imageAttachment.image = UIImage(systemName: "checkmark.circle")
// If you want to enable Color in the SF Symbols.
imageAttachment.image = UIImage(systemName: "checkmark.circle")?.withTintColor(.blue)
let fullString = NSMutableAttributedString(string: "Press the ")
fullString.append(NSAttributedString(attachment: imageAttachment))
fullString.append(NSAttributedString(string: " button"))
label.attributedText = fullString
Result: