I opened a spot position on margin, but why is there no change to my currency balances?

If you open a spot position on margin, there will be no debits or credits to your account “balances” tab, aside from the fees associated with the trade (including the fees specific to spot transactions on margin). Instead, assets you receive when trading on margin will be reflected in your open “positions” tab until you have satisfied the obligations under your margin extension by closing your position through position settlement or one or more closing transactions.

You can find more information about the distinction between your “balances” and “positions” tab here.

The availability of margin trading services is subject to certain limitations and eligibility criteria.