If you click on the Signing for "Unity-iPhone" requires a development team error, a hint will appear instructing you to select a development team in the project editor. To correct this, perform the following steps:

1. Click on the project name at the root of the error tree on the left-hand side (in the preceding case, that's Unity-iPhone).

2. Then, in the main area toolbar, select your project name in the leftmost Targets list (as shown in the following screenshot, that (also) is Unity-iPhone):

3. Select the General tab, then look to the Signing section and choose a team from the select list, as shown here:

4. If you do not yet have a team in the list, you probably have not yet configured your Apple ID with Xcode. In that case, first go to Preferences | Accounts and add an Apple ID by tapping on the plus sign.

Now, once again, try to build by pressing the Play icon in the upper-left corner of Xcode (hint: the Play button's tooltip will say "Build and then run the current scheme").