I use the same command as yours, and repo sync the 4.14.78 successfully without errors., it seems that your network connection issue, could you open https://source.codeaurora.org/external/imx/meta-fsl-bsp-release on the linux environment? try to delete imx-yocto-bsp and bin folder, then build again accroding to i.MX Yocto Project User's Guide of 4.14.78 version