I am not sure which client you are using, but basically for iOS code signing you need two things - a certificate (cryptographic identities) and a provisioning profile connected to it. In the AppBuilder options, there are two sections for the two needed files. I have attached an image showing this.
For the cryptographic identities you need to import a p.12 file of a certificate if you have one. If you don't have one, you must create a certificate signing request or a .csr and create a certificate from it in the iOS dev center. This process is explained here
On the provisioning profile section, you need to provide a .mobileprovision file that is connected to the certificate you uploaded. They must match.
I hope this information is helpful.
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