I can't start the backend

Port is not defined when I try to set up the backend from the start of the networking section.
I Haven’t touched anything other than my IP

Error: Configuration property "port" is not defined
    at Config.get (C:\Users\user\...\Node\Backend\node_modules\config\lib\config.js:182:11)
    at Object.<anonymous> (C:\Users\user\...\Node\Backend\index.js:31:41)
    at Module._compile (internal/modules/cjs/loader.js:1063:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
    at Module.load (internal/modules/cjs/loader.js:928:32)
    at Function.Module._load (internal/modules/cjs/loader.js:769:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:72:12)
    at internal/main/run_main_module.js:17:47
in windows set PORT=xxxx
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