It means that EtceteraManager.cs is referencing the class AbstractManager, however AbstractManager doesn't exist for some reason or another. It's likely that the AbstractManager script is either missing or possibly in Standard Assets when it should be in Assets, or Assets when it should be StandardAssets.
Bottom line is, Unity is unable to find a script called AbstractManager which should have come with your packages, and therefore other scripts referencing said script throw errors. :)
Hope this helps,
Klep
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