Inheritance tax is normally payable by the end of the sixth month following that is which the person died. So, for example, if someone died on 4 April 2020, any inheritance tax due on their estate would be due by 31 October 2020.
Often the deceased estate will include non-cash assets, such as property, shares and suchlike and the beneficiaries may need to sell some of the assets to realise the cash with which to pay the inheritance tax bill. The tax system recognises this and allows the inheritance tax on assets that may take some time to sell to be paid in instalments.