There are basically three possible reasons for this error. I can’t tell which one applies because you didn’t really post any details in the question.
One, your secret code has a subtle bug that wasn’t tripped with previous listings. Resolution: Debug your code.
Two, eBay made a change to their postage policy or listing validation code, and your listings are no longer valid under the new rules. Resolution: Update your code to comply with the latest changes.
Three, eBay updated their servers with code that contains a bug in the logic for validating postage policies. Resolution: Wait for eBay to fix the bug.
solved Error: You’ve provided an invalid postage policy while adding FixedPrice Listing [closed]