Unreal engine is crashing when i try to render and states the following;
Assertion failed: BoneTransforms(I).Bonelndex >= Lastindex [File:D:\build++UE5\Sync\Engine\Source\Runtime\Engine\Public\BonePose.h) [Line: 741)
any help would be appreciated! 
Hello there @Subie_Rebirthh!
The error you are getting is quite particular, a skeleton mesh in your scene has an error in their bones’ hierarchy. UE is most likely trying to calculate a pose of a Skeletal Mesh, but it finds the bones in an incorrect order.
I would suggest isolating any active characters in your level, and check their bones’ structure. You can test by rebuilding the skeletion inside Unreal itself, or if it’s an imported model, you can double-check in Blender or another similar platform.