title: error XA0000: Could not determine API level for $(TargetFrameworkVersion) of 'v7.1'
body: <p>While building the solution , xamarin throws this error. </p>
  
  <p>error XA0000: Could not determine API level for $(TargetFrameworkVersion) of 'v7.1'.</p>
  
  <p>How to solve this error?</p>