This error was generated because a line item referenced in the constraint referenced multiple levels of non-dimensionalized line items. Optimizer works best when relationships are defined in lists and parent hierarchies with dependencies limited to one level removed. Modeling with parent hierarchies and lists items will allow your input line items to be defined by simpler formulas -- let Anaplan do the work!
You can troubleshoot this error by temporarily removing the formula from the line item and running the Optimizer action. If Optimizer is then able to solve the problem, the issue can be isolated to that line item and you can look at simplifying/flattening its formula.
In our case, a second error came up in how the syntax of a constraint was phrased. After changing the syntax, the Optimizer ran successfully.