Error : Failed to login: null

[Schlosi]

Hello, i need help, if i want to connect to MCSG server minecraft says: ,,Failed to login:null" please tell me how to delete this problem. I have capture :

 

[BlackHead]

First of all, do you have purchased MC account?
If no, you will have to buy it if you want to play on this server.
If yes, I suggest you reinstalling your Minecraft or Force-Updating it before login.

 

[Schlosi]

okay thanks and yes i have purchased MC account.

 

[Stunt_PIg]

I use to get this message when I hadnt bought minecraft