question about arithmetic operator '+'

You get the error, because you are using unsuitable data types and incompatible libraries.

There are two possible ways:

- You can refer to the legacy non-IEEE STD_LOGIC_UNSIGNED library. But then you have to import STD_LOGIC_ARITH instead of NUMERIC_STD. bit_vector has to be changed to std_logic_vector in addition

- You can use correct VHDL types and the official IEEE.NUMERIC_STD library. The involved signals should be defined as unsigned in this case.

As a side remark, the inplemented left-shift can be simply written as

var9 <= var3 & '0';

without involving numeric types.