Most of platforms allow each item to have maximum 100 variants only. When you create discounted up-sell or cross-sell offers, there will be many new generated variants. Sometimes you cannot apply the discount to specific bundle or item due to this restriction. However, how many variants will be generated?
With a discounted up-sell offer, the number of variants doubled. Let's take product X, which has 5 variants as an example, the new discounted variants will be 5 too, so the total will be 2x5=10 variants.
When you create a Cross-Sell bundle and offer a lower price for it, each variant of each product included in your Cross-Sell will have additional variants relevant with each variant of the remaining items. For example, your Cross-Sell offer has 2 product items: A & B, A has 6 variants and B has 15 variants. The new variants generated are 6x15=90, the total variants of A are 90+6=96 and B's are 15+90=105 (> 100). Thus, the variants of B will be 105 and more than 100 variants limited by the platform. That's why you cannot complete creating the offer.
However you can use Sales Motivator to apply the discount, this feature will not generate any additional variants, here is the instruction for your reference.
Any further question can be sent to email@example.com.