The backstop in the European Union (Withdrawal Agreement) Bill is a “Single Customs Territory” over which the EU has control of the tax wall around the UK (the “Common Commercial Policy”, I think?).
Under international trade rules, preferential trade agreements must substantially eliminate tariffs between the two parties.
Does this mean that the UK will be prohibited from striking trade deals in the backstop scenario?
The UK will be able to negotiate deals, but would not be able to sign them once negotiations are concluded. It would have to wait for the backstop to end.
In practice it is unlikely that major economies would be interested in negotiating when there is no way to know when any deal would come in to effect and no way to know what the UK's final agreement with the EU would be. Also those countries would be minded to wait until later when the UK is in a weaker, more vulnerable position before starting negotiations from a position of immense strength.
That is also why the UK has failed to secure any major trade deals ahead of the self-imposed March 29th deadline for leaving the EU.
