To import leagues from ESPN you must use a supported desktop browser such as Google Chrome, Microsoft Edge or Brave and have the Fantasy Points Browser Extension (FPBE) installed. The extension provides the bridge between ESPN and Fantasy Points that is required to authenticate and import leagues. Once you have FPBE installed you need to be logged into ESPN and then visit the FLI import page to start an import.
Only the initial import must be done within one of the supported browsers with the extension is installed. Syncing an ESPN league does not require the extension.