- Have you checked if you created the bitcoin.conf file on the right path? It should have been created in .bitcoin/bitcoin.conf
- Within the configuration file, what value have you put after the prune variable? Make sure that you save the file after done editing the config file.
Also note that while you will end up running a pruned node, the client still downloads the full blockchain but it prunes the data (according to the value in the prune variable) as it goes along so make sure your VPS has enough bandwidth for the operation to happen. As for guides, albeit written in 2019, I believe these[1] instructions are still valid.
[1]https://medium.com/@retprogramisto/how-to-install-a-pruned-bitcoin-full-node-on-ubuntu-vps-2b81fe170ddf
