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1. How to enable pruning
Are you using the bitcoin-qt with GUI . If yes you can just do this on the GUIFor bitcoin-qtwith GUI go to your
settings>options
You should see your prune settings like this
Edited out
If not you can also set it using the bitcoin.conf just as it's been mentioned
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2. Is it necessary to run the full node?
Not really but if you have the storage you can just run you full node to save you from redownloading the blockchain again when you encounter Error like this