Problems with proteus simulation of register
The 4051 is an analog multiplexer. Using it for digital signals may work, but isn't recommended. Consider using a part along the lines of the 74xx153 instead. There is no reason to use 4000 series parts in this day and age; they've been obsolete for decades.

On these multiplexers, you're pulling the INH (inhibit) input high. This disables the analog switches entirely; any behavior you're seeing is likely due to floating-bus behavior. The INH input must be pulled low to enable the switches. (But, as I mentioned before, you should really use  a digital multiplexer, not this analog one.)

The VEE error you're getting is related to this being an analog part. VEE is supposed to be a negative supply voltage. Proteus ignores this, but real ICs will require it.