This plugin (mssql_connection) is only for Android, so I assume you're running your code on iOS, which causes the issue

Look at the image, you can see that this package only has android folder, with native Kotlin language, so it doesn't have any Swift code to handle the native method connect on iOS side

If you haven't known about platform-specific code, quickly explain: that some libs need to call to native side (Android & iOS native code) to access some native platform feature, which can't be done in Dart code, and mssql_connection is the one like that