Do you want to have a double sided A5 layout? THen you should change your document set up to A5!
Guides to divide an A4 to 2 A5 is not the proper way.
@jshndsd91 – Willi is right on this.
But if you really want a guide at the center of the page, do the following:
1. Double-click the "cross-hair" where the horizontal and vertical rulers meet to reset the zero point
2. Drag a guide out of the vertical ruler, do not de-select the guide
3. Set the x-position of the guide to "297/2mm"
(InDesign will do the math for you regradless the current meassurement units.)