This top-up amount is not available
Top-up start rejected an amount that is not currently offered (balanceTopUp.amountNotAvailable / 400).
Why this happens
Stale UI after admin removed/changed presets, or client sent a custom amount that is no longer allowed. Distinct from Select a valid top-up amount (empty/invalid selection).
Diagnose and fix
1. Refresh the top-up page; pick a listed amount.
2. Admin: review Balance top-up presets and republish valid prices.
3. Retry after presets save.
4. See Account balance top-up.