Error code 16666100 means TikTok is blocking the product from being added to your showcase. The error comes directly from TikTok, not from Soco.
Why this error appears
A few different things can trigger it:
The product is no longer available. It may be out of stock, removed by the seller, or deactivated on TikTok Shop. TikTok then prevents it from being added.
You are using a Seller account. Affiliate products can only be added on a Personal (Creator) account. A Seller or Official Shop account can only promote its own shop's products and has no access to the affiliate Product Marketplace, so TikTok blocks the addition.
Your TikTok Shop permissions changed. If TikTok restricted your affiliate permissions, issued a warning, or temporarily removed account access, you may not be able to add products until it is resolved. Check your TikTok Shop Creator Center for any warnings or account notices.
Your showcase is full. TikTok limits a showcase to about 2,000 products. If you are near the limit, remove products you are no longer promoting and try again.
What you can do
If you see this error, work through these checks:
Confirm the product is still active on TikTok Shop.
Make sure you are on a Personal (Creator) account, not a Seller account.
Check for any TikTok Shop warnings or notices.
Check how many products are in your showcase.
If the issue continues, let us know and we'll help. Please include the product ID and a screen recording of you trying to add the product in Soco.
In short
Error 16666100 is TikTok blocking a product from your showcase, usually because the product is unavailable, you are on a Seller account instead of a Personal (Creator) one, your affiliate permissions changed, or your showcase is full. Fix the cause and try again, and reach out with the product ID and a screen recording if it persists.