Actually, that sounds like you checked the "Addressable" checkbox on the Pool Manager welcome window, but don't have Addressables package installed from Unity Package Manager. There's a warning about enabling packages you don't have installed on the welcome window that tells you some of this information below.
Anyway, go ahead and install Addressables package from Package Manager. Or uncheck the Addressables support checkbox in Pool Manager welcome window to remove it. Either one will get you compiling again.