Select a valid top-up amount
Top-up start rejected the amount (balanceTopUp.invalidAmount / 400 Select a valid top-up amount.).
Why this happens
No amount selected, custom amount outside allowed presets, or stale UI after admin changed Balance top-up presets. Distinct from Each balance top-up amount must be a valid price (admin preset validation).
Diagnose and fix
1. Pick a listed preset (or a valid custom amount if the UI allows).
2. Refresh the top-up page if presets were just updated.
3. Admin: ensure presets under Payment / balance top-up are valid prices.
4. See Account balance top-up and Balance top-up presets.