The calc() CSS function can be used anywhere a <length>,<frequency>,
<angle>,<time>,<number>,or <integer> is required. With calc(),you
can perform calculations to determine CSS property values.

你不能在那里使用auto,因为它不是calc()的有效值.

calc()的语法

term
  : unary_operator?
    [ NUMBER S* | PERCENTAGE S* | LENGTH S* | EMS S* | EXS S* | ANGLE S* |
      TIME S* | FREQ S* ]
  | STRING S* | IDENT S* | URI S* | hexcolor | function | math
  ;

有关更多信息,请参阅Docs

正如您所评论的那样,您希望将div居中,但您还希望右侧的5px边距比使用text-align：center;在父元素上并使子div元素显示：inline-block;

Demo

产量

div.wrap {
    text-align: center;
}

div.wrap div {
    display: inline-block;
    height: 100px;
    width: 100px;
    color: #fff;
}

div.wrap div.with_margin {

    margin-right: 5px;
    background: #f00;
}

div.wrap div.without_margin { 
    background: #000;
}