I have integrated 1 plugin in my project which is facebook SDK v 7.9 & unity 5.3.5f1
& I get these errors
I’m struggling with this issue for 2 weeks & I had to start from scratch to figure out that the facebook sdk is the source of the error
here is the folders structure inside unity
Happened to me also when dealing with unity ads.
Search for .jar and .aar files in your unity assets folder, if you have multiple files with the same name, you should delete (or temporarily rename) one of them. For me, it was Classes.aar
This kind of errors may occur because of many reasons. The very first thing which I have seen and want you to fix is your Android SDK. It seems like too old version of the SDK, so please download the latest Android SDK and then updates the Tools, API and etc.
If issue can’t fixed by above solution, take a backup of the project, Right click inside Asset folder and click on reimport all.
I think this will fix your issue, if you still have any problem Please let me know. I will be happy to guide you further more.