Cypress custom command is not recognized when invoked

All the code and referenced modules in index.js are loaded before your test file. So you need to refer(require) commands.js in your index.js file. You can however import commands.js module directly in your test file but then you need to include it every test file. Recommended approach is to include it in index.js file and you are not worried about explicitly refer in your test files.

To expand on @Dinesh Kumar's excellent answer, it's also important you haven't disabled support for the default support file (I know, unfortunate naming scheme in this case) inside your cypress.json by adding the line: supportFile: false.

Delete that line from your cypress.json if it's there. You can also specify a different path if you're not happy using the default path of cypress/support/index.js.

Working index.js with commands.js file - both in the support folder:

// index.js
const customCommands = require('./commands.js')

module.exports = {
  commands: customCommands
}

And double check your settings:

// cypress.json
{
  "baseUrl": "http://demo.your-domain.test",
  "supportFile": false,  // <-- delete this line present
  ...
}