Hi
I think this is because of Betfair's SP minimum
copied from Betfair SP rules:
The minimum liability for a bet placed at Betfair SP is £10 for a lay bet and £2 for a back bet (or the currency equivalent in each case)
so you can only place a pre-off SP bet using a liability
or, you could lay at actual_sp as soon as the market turns in-play, but no guarantee of getting matched.