You will not be able to use the Functional Skills Exemption Evidence part of the Functional Skills Onboarding Wizard step without an API.
Without an API to a functional skills platform, the only way you can upload documents into the Functional Skills section of a learner's record is to use the Functional skills section within a review. Within this review section there is an 'Exemptions' section which you can use to upload the exemption files.
Once you select an exemption status, then you will get the option to upload a file.