You are only defining precedence rules for pseudoterminals (PostDecrementation and PreDecrementation). Nowhere do you define the precedence of the actual terminal Decrementation.
Precedence is always compared between a production (which could be reduced) and a terminal (which could be shifted). So you need to specify the precedence of Decrementation to resolve the conflict.
Personally, I wouldn't use precedence declarations in this case. But if you really want to, you should probably just use one pseudoterminal, and let Decrementation be the precedence of one of the two productions which uses it.