I have a very annoying problem, when I try to open an imported project with my version of Eclipse, it tells me that I can’t do it unless I download the version 21 of something.
Then it opens my SDK Manager and when I want to install the new packages to update my version, it’s just impossible even after having accepted the terms and conditions.
Here is a picture :

Any help is welcome!
Thanks in advance.
try First Clear your cache with
Android SDK Manager--> Tools--> Option-->click on Clear cacheafter Refreshing the List byAndroid SDK Manager --> Packages --> ReloadHope it will Help