To uninstall apps that are locked and were already on your system when you bought it new, you may need to root your phone first (which essentially makes you an admin user able to do anything you want to your phone), but some pre installed apps can cause problems with your android phone if removed. What app is it?.
If you installed this app yourself, try scanning it for spyware and removing it using a security antivirus tool such as bitdefender, or uninstalling it via a security app such as cleanmaster and it's 'appmanager'.
Be careful whilst using cleaning tools, as if you are not careful, you can easily delete your contacts and messages if accidentally checked. It is best to leave pre installed apps such as google services alone.