Generate Code function not found

Generating Transfer Code
Use the Transfer Settings > Generate Code command to generate a code that automates the transfer. The Generate Transfer Code window opens with the generated code.

The section, which you are quoting, is from Transfer options dialog documentation:
https://winscp.net/eng/docs/ui_copy
Not from "Keep remote directory up to date" dialog. That dialog does not have code generation functionality.

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Instead, generate a basic script template for your session:
https://winscp.net/eng/docs/guide_automation#generating
And add keepuptodate command:
https://winscp.net/eng/docs/scriptcommand_keepuptodate