Owing to a lack of further information, this is purely a guess with regards to your problem. However, the functionality of Implement (Crtl + I) and Override (Ctrl + O) depend on if your class is implementing an interface or extending another class.
Implement (Ctrl + I)
This will only work if your class is implementing an interface or extending a class with abstract methods e.g.
public class MyClass implements MyInterface {
Pressing Ctrl + I will bring up a list of methods you have yet to implement. If you have implemented all methods then this will bring up the "No methods to implement have been found" error as there is nothing else to implement.
Override (Ctrl + O)
Pressing Ctrl + O will bring up a list of methods from your parent class that you can change the behaviour of (i.e. override the behaviour). By default this should bring up the public methods of Object as all Java classes implicitly extend Object by default. I can’t say why you’re getting this error unless you post the class that you’re trying to override in your question, my only guess is that you’re extending a class with no override-able methods.