Splice credits work differently in Splice Create, depending on which plan you're on:
Create Pro plan
Sample licensing does not use Splice credits
Samples are licensed to you upon project export
Creator & Creator+ plans
Sample licensing uses Splice credits
Credits are charged upon project export OR after adding a sample to a project -- use the "license samples when you use them" toggle in the preferences to select your preference
Sounds+ plan & all other Splice plans
Not possible to spend credits to license Splice samples through Create
Project export in Create is not available for this plan