Olá, Leonardo
Executei exatamente o mesmo comando na minha aplicação Xamarin:
PM> Install-Package Acr.BarCodes -Version 3.0.0
Attempting to gather dependency information for package 'Acr.BarCodes.3.0.0' with respect to project 'TestDrive.Aula6', targeting '.NETPortable,Version=v4.5,Profile=Profile259'
Gathering dependency information took 1,34 sec
Attempting to resolve dependencies for package 'Acr.BarCodes.3.0.0' with DependencyBehavior 'Lowest'
Resolving dependency information took 0 ms
Resolving actions to install package 'Acr.BarCodes.3.0.0'
Resolved actions to install package 'Acr.BarCodes.3.0.0'
Retrieving package 'Acr.BarCodes 3.0.0' from 'nuget.org'.
GET https://api.nuget.org/v3-flatcontainer/acr.barcodes/3.0.0/acr.barcodes.3.0.0.nupkg
OK https://api.nuget.org/v3-flatcontainer/acr.barcodes/3.0.0/acr.barcodes.3.0.0.nupkg 165ms
Installing Acr.BarCodes 3.0.0.
Adding package 'Acr.BarCodes.3.0.0' to folder 'C:\Users\marce\Documents\GitHub\xamarin-crie-aplicacoes-mobile-parte-2\packages'
Added package 'Acr.BarCodes.3.0.0' to folder 'C:\Users\marce\Documents\GitHub\xamarin-crie-aplicacoes-mobile-parte-2\packages'
Added package 'Acr.BarCodes.3.0.0' to 'packages.config'
Successfully installed 'Acr.BarCodes 3.0.0' to TestDrive.Aula6
Executing nuget actions took 4,29 sec
Time Elapsed: 00:00:05.8419260
Porém, instalou com sucesso, sem erros.
Você pode tentar de novo e colar aqui no fórum todas as mensagens de log que aparecem na janela do Package Manager Console? Obrigado!