Conducting this here because none of the other answers really went in enough detail to help me, and I still had to figure it out on my own.
On the Apple Developer website, log in and click "Under" under the certificates. The certificate must be generated on YOUR machine, and not build with your account or things. That's what confused me ... You can click the + icon and create a new certificate (development or distribution). If it does not allow you to select, for example, Distribution, this is because there are already too many certificates in the account. So, first you need to delete the old certificate.
1) Delete the oldest certificate on the portal
2) Click + and create a new certificate - this will help you open access to Keychain Access on your computer and request a certificate from Apple, and you will need to save the file and upload it to the developer's portal to create a certificate.
3) Upload a new certificate and it will add access to Keychain
4) Go back to the preparation profiles on the Dev portal, press EDIT on the one you are trying to use, and change it to use the newly created certificate.
5) Download and click preparation profile
6) Everything works fine.
This process was not really so complicated, but for some reason I could not find the right explanation anywhere. Hope this helps someone.
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