Please see the comment discussion above -
Specifically regarding the "QMixed" error, that's an error which occurs when trying to make an IQTensor (usually an operator returned from the .op method of a site set) which would violate the QN symmetry in the sense of not having a well defined QN "flux" (http://itensor.org/docs.cgi?page=book/block_sparse). The purpose of QMixed storage is to "smuggle" an ITensor (non-QN-conserving) out of a function that only returns IQTensors. It's kind of an ugly workaround and we are planning to fix the design issue that led to it a different way in the future.
So then there are probably two things causing the error you're seeing:
1. you are making an IQTensor (operator) which doesn't conserve the quantum numbers it needs to. For example, you are trying to make the operator Sx as an IQTensor in a context where total Sz is conserved (solution: use S+ and S- instead).
2. you are working in a context where you aren't conserving any quantum numbers, in which case you need to convert the IQTensor with QMixed storage to a regular ITensor before using it.
If you're making a custom site set, please follow the pattern used to make operators such as Sz, S+, and S- in spinhalf.h for making most of your (QN conserving) operators, not the pattern for making operators such as Sx or Sy which are special cases.