Adjust SDK upon launch is trying to read referrer information from Google Referrer API if it's present in the app. This message points out that SDK didn't manage to read that information. It's because you didn't add
installreferrer library to your app like described in this chapter. Of course, if you're distributing your app outside of Play Store and are not interested in collecting Google Advertising Id, then you don't need to add this dependency to your app and error message will be there, but it's causing no harm at all, so you can feel free to ignore it.
In case you have any further questions, feel free to ping.