Reason One
The selected type was selected incorrectly.
Workaround:
On the left there are two categories, one is keychain, one is kind, to select my certificate or certificate in the category. Then in the list of certificates on the right, right-click to export.
Reason Two
There is a problem with the certificate generated with the keychain in the format (certsigningrequest)
Workaround:
Keychain Access--Certificate assistant--Request certificates from a certification authority ...
Reason Three
The first two reasons to ensure that there is no problem, it is possible that the Mac system is missing Applewwdrca.cer certificate.
Workaround:
This certificate is Apple's official certificate, Apple World Wide Developer Relations Certification Authority
: Apple PKI
I met this problem by reason one solution. But see still someone is gray, the article continues to update in ...
Solutions to problems with Apple's inability to export p12 certificates.