You may have reached the daily limit set by Shopify. Stores with 50,000 or more variants are subject to a daily rate limit for uploading variants by using an app or CSV file import. You can add up to 1,000 new variants in a day before the limit is reached. If you exceed this limit, then an error is displayed and you need to wait 24 hours before you can add variants by using an app or CSV file import. Shopify Plus stores are exempt from this limit.
You can read more on Shopify's official knowledge article