Can I terminate SPA for property still under construction? It is not stated in SPA.

i think you can but you need to check if you need to pay a penalty.

Normally, SPA does state the penalty related to to the purposely cancellation by either party, condition of it etc.

It won't be leave "blank" area that openly may be abused by either party.

Got in touch with developer, they try to impose the non payment clause which is unfair so still negotiating.

Just wondering, is your SNP involved mortgage from bank? If yes, this will be a very messy situation, it is best you engage a lawyer to help sought out this issue, because it involves your bank, the developers and the developers bank and lawyer. If previously the lawyer fee, stamp duty is absorded, now you will need to repay it back. There is also other penalty charges mainly in the form of interest charges by both side.

No, cash so I am dealing direct with developer.