Hi Toms,
It sounds like a mismatch between how Shopify is expecting the metafield data to be formatted, and how your script is preparing it. Without seeing your script it’s hard to know what the issue is, but “Owner” in this context would be the object class that the metafield belongs to, e.g., “Product”, or “Customer”.
More on metafield constraints here: https://shopify.dev/docs/apps/build/custom-data/metafields/conditional-metafield-definitions
Don