We can technically strike trade deals during transition, however, they won't be able to take effect until after transition. The problem is we won't know what our future relationship will be with the EU until the end of the transition period. Plus the backstop is going to be the minimal level our of future trading arrangement so any trade deal with another country won't allow us to amend the tariffs set by the EU, therefore taking away 90% of our leverage. The other 10% of leverage is the quality of goods we'll accept from third countries. The issue here being that any acceptance of goods that don't meet EU standards will make frictionless trade with the EU impossible as they will have to implement checks on the border.
As for the FTA's we have via the EU, they all end in March 2019. After that any of these third countries are under no obligation to honour the existing terms of our deal with the EU. Furthermore, any new trade deals between the EU and third countries will not include the UK so we will not necessarily benefit from any of those.
The real issue is that we're caught in the backstop. We're tied to the EU Customs union for goods but have no guaranteed access to the single market. That means on day 1 they can demand any amount they want from us for access knowing that we're tied to the CU so can't do any meaningful trade deals of our own so they are pretty much our only option for trade. This deal leaves us entirely beholden to them and they know it.