If you set volume weighted each species works out it's own density and then the mixture density that's used in the flow equations is calculated from those. If you set some of the other mixture level options you'll find Fluent requires different inputs at the species level.
The CAR:CDR error means something somewhere isn't set. So, I suspect during a species/mixture change you've managed to get a field with no value. When you initialise the mixture will be set based on the initial conditions. No need to patch unless what you chose isn't what you wanted.
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