itunes not able to sync flac

Apple has never supported and likely never will support FLAC in either iTunes on a Mac or PC, and iTunes on iOS devices or iPods.

Since FLAC is a lossless format and Apple's own ALAC aka Apple Lossless Audio Codec is a lossless format and since both are exactly the same audio quality, both are roughly the same file size and both have the same meta-tag capabilities you should use a utility to convert your FLAC tracks to ALAC.

Note: Windows Media Player for Windows 10 fully supports using ALAC and FLAC so from that point of view there is no reason not to switch to ALAC. Many tools on Windows support ALAC e.g. Windows Media Player, Microsoft Media Center, dbPowerAmp, WinAmp, VLC, Kodi.TV, Plex, Logitech Squeezebox Media Server, Sonos, etc. etc.

Where Windows is a bit week is in having good free tools to convert between FLAC and ALAC. I use XLD aka X Lossless Decoder but this is a Mac tool.

With specific regards to using a lossless audio format on an iPod. Yes iPods except the iPod Shuffle do support using ALAC but not as mentioned FLAC, however the extra audio quality provided by a lossless audio format is generally considered a waste of time when listening via the ear pods or ordinary headphones on a portable media player e.g. iPod or iPhone. Considering the extra audio quality cannot for most people be actually heard the much larger file size on devices like these which have limited storage capacity is wasteful. Therefore using MP3 at 320kbps or AAC at 256kbps or better is normally a more suitable choice. iTunes can be configured to automatically convert ALAC to AAC when syncing to an iPod and this does not result in duplicate copies in your iTunes library, it also only has to do this for tracks that change between each sync so normally this is a smaller amount.

Apple don't support the use of .flac audio files. You can convert all of your files en masse using a third party program before importing them into your iTunes.