my android phone cannot connect my modem router it is showing authentication problem

[Aman Gazal]

authentication problem in my lenovo A6000y

 

[Phillip Corcoran]

Authentication error is often the result of typing the wireless key incorrectly, or typing some of the letters in the wrong case, because wireless keys are case-sensitive.

So open your router's configuration utility and double-check the wireless key in the wireless settings.
When entering that key on your phone, you must type it exactly as it appears in the router config utility.