Because there is only one tail here, is this reason for the error tag?

I say no. There is no indication whether or not her human form has any tails at all in canon.

I like ZUN's original design to replace Orin's two cat tails with braided twintails.

There could still be tails hidden under her dress, but they're not shown or mentioned anywhere. On the other hand, Kyouko was confirmed to have a tail in SoPM (text, no image).