Suggestions

That solved it! Thank you SO much, Danebi. My son will be thrilled in the morning. He's been sad without access to some of his worlds for a few weeks.

OH, I see the mods folder. Just delete that mod file?  

Is that a mod I should remove? ... and how do I do that?  

repurposed structures is for 1.16.1.

Okay, so I'm trying to work out how to detect if there is a block in the shaded red area. Thanks to the massively helpful vemerion I'm now trying to raytrace this area, but I'm struggling getting it to work.   Does anyone have any tips/pointers? I'm not really sure how to set up World.rayTraceBlocks(Vec3D(), Vec3D) as whenever I put in the Player's location for one vector and the player's location + a value for the other it doesn't work.   Here's what I'm trying to achieve, where X and Y are both configurable lengths, this is really throwing my head so thanks so much for your help!