Swift – Xcode 6.1: file was built for x86_64 which is not the architecture being linked (i386)

You may have a bad connection in your xib.

I've had this error many times. While TechZen's answer is absolutely right in this case, another common cause is when you change the name of a IBOutlet property in your .h/.m which you've already connected up to File's Owner in the nib.

From your nib:

1. Select the object in IB and go to the 'Connections Inspector'.

2. Under 'Referencing Outlets' make sure that your object isn't still connected to the old property name... if it is, click the small 'x' to delete the reference and build again.

   

Another common cause if you are using Storyboard, your UIButton might have more then one assignings (Solution is almost the same as for nib):

1. Open your storyboard and right click the UIButton

2. You will see that there is more than one assign/ref to this button. Remove one of the "Main..." greyed windows with the small "x":

   

After struggling with this same problem and following all the accepted answers of updating build settings, clearing the linker search path, etc.. I finally discovered an answer that worked for me.

Before building, make sure you select right type (iPhone Simulator) instead of iOS Device. Then rebuild. Otherwise, you're trying to use a library built for an iOS device (arm processor) on a simulator on your mac (i386). Should've been obvious, but wasn't.

Before:

After:

Now, look in the Products group in the Navigator > right click your static library (.a file) > Show in Finder, you'll notice that its in a Debug-iphonesimulator folder instead of Debug-iphoneos. I didn't pay any attention to the folder name originally, or I might have thought of this sooner.

Hope this helps.