/alter in now an admin endpoint in dgraph v20.07.x and so it requires IP whitelisting. You need to use --whitelist flag to bring up dgraph alpha in v20.07.x. Use that flag to specify a list of whitelisted IPs which can communicate with the alpha server. See here for how to do that. See here for a list of endpoints which are now admin endpoints and require IP whitelisting with several other things.
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