Bulk Product Edit uses Shopify's API. For tasks which create new products/variants, they could encounter a daily variant creation limit. The following screen shot from https://shopify.dev/concepts/about-apis/rate-limits states when this limit will apply.
1. It is a non-Shopify Plus store.
2. The store already has 50K variants.
3. There are 1000 variants created already in a day. (Store wide. Cross all apps. )
If Bulk Product Edit encounters such error message from Shopify API call, it means your store has reached that limit. The running task will be throttled (paused temporarily). Throttled task will be resumed automatically 24 hours later. Because 24 hours later, the store can create 1000 new variants again. Based on our observation, even if a store has reach 50K variants, it maybe able to create more than 1000 new variants per day. It could be that Shopify is often not strictly enforcing this rate limit.