This error usually occurs when Gradle cannot find a particular component. I experienced this when trying to use the Salesforce React Native Android example.
The error message you get looks like the one you posted … in my case it was
Project :app declares a dependency from configuration 'compile' to configuration 'default' which is not declared in the descriptor for project :libs:SalesforceReact.
I found my Gradle settings file ( called settings.gradle ) in the projects android directory. The start of it looked like this
rootProject.name="SmartSyncExplorerReactNative"
def libsRootDir = new File( settingsDir, '../node_modules/SalesforceMobileSDK-Android/libs' )
include ':app'
include ':libs:SalesforceReact'
project( ':libs:SalesforceReact' ).projectDir = new File( libsRootDir, 'SalesforceReact' )
In this case, the path given in libsRootDir did not exist (as these libraries actually came from a different repository which I cloned and then pointed this path to!).
Once I had corrected the path, save the settings file and reran, everything worked smoothly.
I hope this helps,
Jonathan.