I ran into this today and found this tidbit in the TikTok documentation (https://ads.tiktok.com/marketing_api/docs?id=1767309272869890&rid=68wfzvwt9ov):
I could not call the /v1.3/ad/create/ endpoint in the sandbox environment and the response said an identity_id and identity_type is required.
identity_id and identity_type are required in /v1.3/ad/create/, but currently, the sandbox doesn't support the identity related endpoints. As a workaround, you can use the /v1.2/ad/create/ endpoint instead, which only requires the identity_id and identity_type parameters when creating Spark Ads.