"For some reason", Here's the reason (the link that's specify on website are not same with link that's used in eclipse on website as Guide).
Not same, I mean, not about github.com, I mean about l2j_server.git & l2j_datapack.git
And here's the fix.
You must update the links or we've to change it manually from URI: or Repository path: as L2J_Server.git also L2J_DataPack.git