when requesting a quote for Atlassian Access, customers always come back with the question of where they can find it.
You need this number to be able to request a quote.
The help section here : https://confluence.atlassian.com/support/finding-your-support-entitlement-number-229840018.html#HowtofindyourSupportEntitlementNumber(SEN)-Ifyou%E2%80%99reanadministratorwithproductaccess
only gives information how to retrieve SEN numbers for Server and Data Center products.
This link is used though on the SD where one requests a quote for Cloud.. so this is a dead end.
I don't have anything really helpful for you here. Cloud doesn't have the concept of a SEN. During my migration I had both Cloud and Server licenses active and the My Atlassian page only ever showed the SEN for my server products.
Since Cloud works with your Atlassian ID when you go to contact support while logged in, it will automatically populate a list of all of your support cloud products for you to select from. I know this doesn't help you as you are trying to request a quote on behalf of a customer.
I think it's worth following the steps for when you don't have an SEN and reaching out to the Customer Advocate team to explain the issue and they might have some suggestions for you, or at the very least they may be able to get in touch with the right people to help fix the intake process.
I hope that helps a bit.