question

After API level 25 it doesn't matter what permission you have you won't be able to access accounts of other apps, hence your error. See
https://developer.android.com/about/versions/oreo/android-8.0-changes#aaad

We are working on changes that will allow you to target API levels higher than 25 and retrieve the auth tokens you need without the Account object.

Watch this class for changes: https://github.com/clover/clover-android-sdk/blob/master/clover-android-sdk/src/main/java/com/clover/sdk/util/CloverAuth.java

Maybe next year this class will be updated but in the mean time you need set your API target to 25 or lower. Note that just because your target is 25 or lower doesn't mean you cannot invoke APIs on higher levels (of course most Clover devices are lower than 25 anyway).