When a proposal is created and then a split is added, THEN the rep name on the proposal changes to a different rep it will generate a split error stating it does not add up to 100%, even when it does.
For Reps :
Reach out to the PREObot to have a PREO champion fix this for, as you will not be able to make the necessary changes.
For PREO Champions :
You have to change the proposal back to the original rep that created the split to start with, then remove any splits that are not the original rep, then change the proposal back to the proper rep and this will clear the error. To find the original rep, go to milestones>activity and scroll through until you see where they added the split. In most cases, the original rep will be the one to have created the split but occasionally it could be an overlay or another rep with permissions to access that proposal.