Hi. There is no way to understand for which version of Forge this mod is, but the "forge-1.17.1-37.0.25" definitely does not want to play with it.

https://ichun.me/mods/sync/ Sync got from here. It is for 1.17.1, so I doubt it is "ancient"

Starting from Minecraft 1.10.2 onwards, I have always included the Minecraft version in the file name. As you can see from the two iChunUtil jars you have installed (one of which you do not need [and may also crash the game]), those versions are for 1.10.2.

The reason I say those files are "ancient", is because the file for Sync you downloaded dates back to October 2016. It is coming up to about 5 years old soon, which in MC mod history, makes it pretty old.

In case you are confused, I have not updated my website for years. All my mod updates have been posted to CurseForge nowadays. The url you posted states that Sync is available for MC One Seven Ten. Not One Seventeen One.

Sync (and it's latest files) can be found here: https://www.curseforge.com/minecraft/mc-mods/sync/files

Sync is not yet updated to versions beyond 1.12.2. I may or may not get to it in the future.

Hope this clears that up. :)

Obligatory: https://howoldisminecraft1710.today/

Have a good day.

Thank you, understood!