To fix this issue Forms supports Android 4.0.3 (API 15) and higher, you are confusing the "Target Framework" (compileSdkVersion) for the "Target Android Version" ( targetSdkVersion) vs the "Minimum Android Version" (minSdkVersion).
To fix the issue you can do I have found what the issue is. The initial creation of a Xamarin Forms project had some buggy startup operations that broke the referencing from the Android and iOS projects to the .NET Standard Library project. I had to go into each project and re-reference the .NET Standard project to the Android and iOS. I later found out that the default installation were missing certain android SDK files and had to be later installed through the Xamarin Android SDK manager to resolve the referencing issue, as that may have been causing errors during start up (maybe the process was looking for a certain SDK version). I decided to install all versions from Oreo down to Lollipop.
can not create xamarin.forms project from visual studio 2015
around this issue Long story short, to build an .ipa, choose AppStore (or Ad-Hoc, Release) | iPhone as solution configuration and build the project. After build, you'll find the .ipa file in project's output directory (for example, bin\AppStore). Of course, you have to have corresponding provisioning profiles configured in Xcode but this is another question. More here.
Does building a Windows Visual Studio Xamarin.IOS class library create an Xcode project on the paired MacBook?
this one helps. No it does not create an XCode project on the Macbook that you can edit in Swift. It only uses the Mac to compile the code. You have to Install Visual Studio for Mac on the Mac computer Transfer your directory to the mac and Open the same SLN file in Visual Studio for Mac