Hi,
Please note that: In-place upgrade from Windows 7, Windows 8.1, or Windows 10 semi-annual channel to Windows 10 LTSC (previously LTSB) is not supported.
Reference: https://learn.microsoft.com/en-us/windows/deployment/upgrade/windows-10-upgrade-paths
You need to execute a clean-install to install 1607 Enterprise LTSB, you could get the image file from VLSC or MSDN subscription:
Sign in to the Volume Licensing Service Center to download Enterprise editions.
Sign in to your subscription on the MSDN Portal for MSDN downloads.
Then refer to the following steps:
https://support.microsoft.com/en-us/windows/reinstall-windows-10-d8369486-3e33-7d9c-dccc-859e2b022fc7#bkmk_clean_install_of_windows_10_using_installation_media
Best regards.
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