Does it need the entire chain once it builds the index? Can't you get rid of it later? Just wondering why you can't build the index by running pruned node.

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I found this tutorial on the internet and I think that he explains it simpler than on the one of github. I haven't worked on Bitcoin Core a lot, for example I've never used its command line program, and thus I'm getting an error I don't understand. By running ./bitcoin-cli --testnet getinfo on the terminal I should be getting information about my node (such as block height).

I have opened the Bitcoin Core application (using ./bitcoin-qt --testnet) but once I run the above command I get:

What is going on?