Failed to find target with hash string 'androi

问题:

I tried all the solutions here & there on stackoverflow but still getting this error even though i updated the :-

1. compileSdkVersion 27
2. buildToolsVersion "23.0.2"
3. targetSdkVersion 27

Already tried these threads solutions but nothing seems to work:-

Error: failed to find target with hash string 'android-21'

Failed to find target with hash string 'android-25'