What would follow if any of the national parliaments refused to ratify the EU-UK trade agreement?
The only parliaments that have to ratify it are the UK and the European, individual EU countries are not required to approve it.
The odds of either of these rejecting it are very small. In the case of the EU Parliament, if it were to reject it, then it would be asked to vote again and again until it gave the "right" answer. See the various referendums on EU issues over the last two decades (with the exception of Brexit).