You can't advertise on LINE, because LINE doesn't understand the importance of a healthy community.
You can advertise on Tapas. You can advertise on Twitter, Facebook, etc.
You can advertise on places outside of LINE to bring over to LINE, but as far as promoting on LINE, the only you can do is post your work and get views there directly. You're only other option is to literally post in other comments telling people to check out your work but you can probably guess how that would go.
Seriously, its one of just the many problems with LINE and arguably its biggest problem. There is no community on LINE, there is no way to build relationships between readers or even fellow creators. So yeah, thankfully there's other places to help promote yourself to visit LINE but trying to promote directly on LINE is all but impossible.
As mentioned, someone on Tapas started a Webtoon Cross-Promotion topic to help people struggling with LINE, but getting something from LINE directly? Yeah...as the old saying goes...S.O.L.