I wish to sell a parking space (€9,000) and an apartment (€250,000), which is my secondary residence, to the same buyer. Will the parking space be subject to capital gains tax?

Yes. The €15,000 exemption threshold for capital gains tax is assessed per sale and per buyer. According to BOFiP BOI-RFPI-PVI-10-40-70 §80, if multiple lots are sold to the same buyer in a single transaction, the total sale price is considered. 

Therefore, to benefit from the exemption for the parking space, it would have had to be sold separately to a different buyer. Since both lots are sold to the same person, the parking space is not exempt from capital gains tax.