Because of the space in the middle (after EndNormalBracket)
stackPanelOneZeroZeroPercentBasicBuybackStartNormalBracketMotorplusEndNormalBracket StartNormalBracketOneEightZeroOneEndNormalBracketStartNormalBracketUnderwrittenEndNormalBracket
here ^
Other than that, there is (technically) nothing wrong with it.
3
solved Invalid WPF name? [closed]