xamarin.android project deployment errors - deployment

Question

i made a project to try and use admob banner but when i deploy the project on my device (api 21) i get a "there were deployment errors. continue?" massege but when i run it on my emulator(api 23) it works perfectly.
my target android version and target framework version are both set to api 21
this is what i get in the output window
sorry for the length i wasnt sure how much of it to put
:Deployment failed
2>Mono.AndroidTools.InstallFailedException: Failure [INSTALL_FAILED_UPDATE_INCOMPATIBLE]
2> ב- Mono.AndroidTools.Internal.AdbOutputParsing.CheckInstallSuccess(String output, String packageName)
2> ב- Mono.AndroidTools.AndroidDevice.<InstallPackage>c__AnonStoreyE.<>m__0(Task`1 t)
2> ב- System.Threading.Tasks.ContinuationTaskFromResultTask`1.InnerInvoke()
2> ב- System.Threading.Tasks.Task.Execute()
2>The "InstallPackageAssemblies" task failed unexpectedly.
2>System.AggregateException: One or more errors occurred. ---> Xamarin.AndroidTools.AndroidDeploymentException: InternalError ---> Mono.AndroidTools.InstallFailedException: Failure [INSTALL_FAILED_UPDATE_INCOMPATIBLE]
2> at Mono.AndroidTools.Internal.AdbOutputParsing.CheckInstallSuccess(String output, String packageName)
2> at Mono.AndroidTools.AndroidDevice.<InstallPackage>c__AnonStoreyE.<>m__0(Task`1 t)
2> at System.Threading.Tasks.ContinuationTaskFromResultTask`1.InnerInvoke()
2> at System.Threading.Tasks.Task.Execute()
2> --- End of inner exception stack trace ---
2> at Xamarin.AndroidTools.AndroidDeploySession.<RunLoggedAsync>c__async1.MoveNext()
2>--- End of stack trace from previous location where exception was thrown ---
2> at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
2> at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
2> at Xamarin.AndroidTools.AndroidDeploySession.<StartAsync>c__async0.MoveNext()
2> --- End of inner exception stack trace ---
2> at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
2> at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
2> at System.Threading.Tasks.Task.Wait()
2> at Xamarin.Android.Tasks.InstallPackageAssemblies.Execute()
2> at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
2> at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext()
2>---> (Inner Exception #0) Xamarin.AndroidTools.AndroidDeploymentException: InternalError ---> Mono.AndroidTools.InstallFailedException: Failure [INSTALL_FAILED_UPDATE_INCOMPATIBLE]
2> at Mono.AndroidTools.Internal.AdbOutputParsing.CheckInstallSuccess(String output, String packageName)
2> at Mono.AndroidTools.AndroidDevice.<InstallPackage>c__AnonStoreyE.<>m__0(Task`1 t)
2> at System.Threading.Tasks.ContinuationTaskFromResultTask`1.InnerInvoke()
2> at System.Threading.Tasks.Task.Execute()
2> --- End of inner exception stack trace ---
2> at Xamarin.AndroidTools.AndroidDeploySession.<RunLoggedAsync>c__async1.MoveNext()
2>--- End of stack trace from previous location where exception was thrown ---
2> at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
2> at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
2> at Xamarin.AndroidTools.AndroidDeploySession.<StartAsync>c__async0.MoveNext()<---
2>
2>Build FAILED.
2>
========== Build: 1 succeeded, 0 failed, 0 up-to-date, 0 skipped ==========
========== Deploy: 0 succeeded, 1 failed, 0 skipped ==========