If I'm not mistaken, the error is caused by the lack of OpenNSFW, which he began to use in recent works. The only problem is that it is not known which version of the mod was used in this scene.
The author of the mod indicated that the new version 2.0 cannot use sounds from the old version and vice versa, or rather they cannot be used at the same time. Therefore, when I installed this mod, only part of this error disappeared, but in the end it did not disappear completely.
Six may have used an intermediate version between Legacy and 2.0
Here is this mod, it seems to me that in the future Six+ will continue to use it, so it will definitely not be superfluous.
A port of the OpenNSFW SFX Pack for use with Koikatsu Studio - AX-MMD/Koikatsu-OpenNSFW-Sound-Pack-port
github.com
Be sure to read the mod manual, as there is mod variability there.