The Flux Dev model hosted on Hugging Face requires a manual step of acceptance, which prevents it from being downloaded programmatically. As a result, the model cannot be added to your instance through InstaSD.

Solution:
Use the Civitai-hosted version of the Flux Dev model, which does not require manual acceptance. You can find it here:
​Flux Dev Model on Civitai

For detailed instructions on adding models to your instances, refer to this article.