Same issue here, I just installed the mod so I'm wondering if it's voluntary or a bug?

Oh wait, after reading comments, Author mentionned it's probably mod related. I have indeed 12 turn per year activated, which isn't needed since already implemented. I'll try deactivating it.

Yeah, this means that you aren't following the mod's instructions and are using incompatible mods.

Shieldwall is a full game overhaul. Mods which are not intended to be used alongside it will probably cause anything from minor problems, to massive imbalances, to crashes. Unless a mod specifically says it is meant to be used with shieldwall overhaul, then you should not be using it with shieldwall.

Like, we edit more than 150 tables, including basically every game system, and many of them are changed to the point that they don't share any data with vanilla. If you're using other people's mods, expect to have serious issues.

This particular problem is caused by using a mod which edits startpos.esf, which would include turn per year mods. As mentioned by the poster above me, they are also unncessary as shieldwall is 12 turns per year by default. Unlike other total war games, unit mods in Thrones of Britannia also require startpos.esf edits, so those will also break the campaign (on top of being game-breakingly unbalanced due to shieldwall's very different unit stats scale from vanilla).