Have you try also sh -ci "$(curl -fsSL https://smartcontracts.org/install.sh)" ?
The doc has been moved from sdk.dfinity to smartcontracts.org, maybe some urls aren’t valid anymore 
Have you try also sh -ci "$(curl -fsSL https://smartcontracts.org/install.sh)" ?
The doc has been moved from sdk.dfinity to smartcontracts.org, maybe some urls aren’t valid anymore 