This is a bit confusing. When creating a lookup index, the number of shards has to be set to 1. Normally, users can omit this and rely on the default setting - but when the default setting isn't 1 shard, the index creation will get rejected.

Users probably shouldn't have control over, resp. shouldn't have to set, the number of shards of lookup indices, anyway.

The setting is read on creation of lookup indices here.