These errors occur when the mxFIMLObjective has no dimnames for the covariance or means, and cannot find them elsewhere (i.e., in the target algebra). For example:
modelSuggested solutions:

Add dimnames to the covariance and means matrices.
mxMatrix("Symm", 1, 1, T, 1, name="expCov", dimnames=list(selVars, selVars), mxMatrix("Full", 1,1, T, 0, name="expMean", dimnames=list(selVars, selVars)),
 Add dimnames to the objective function. Keep in mind that the objective function does not actually have row and columns names. The 'dimnames' argument to the objective function is a convenient shortcut for adding dimnames to the covariance and means matrices.
mxFIMLObjective(covariance="expCov", means="expMean", dimnames=selVars)