I'm getting an error:
No signing certificate "iOS Development" found
No "iOS Development" signing certificate matching team ID "my team id" with a private key was found.
And all my certificates are missing their private keys. Please help!
I reset my keychain, and it fixed my problem. Thanks anyway!
What happened is that the first time that I tried to run the app on my phone is that there was a Keychain Access box that came up asking for the "login" keychain password which I didn't know, so I clicked Deny, but it needed that password to sign my code. Resetting my keychain let me make a new password, and when I ran my code again, it asked me again.