It's not mandatory for a client to upload documents if they can't find them. The language on screen encourages upload but it's not mandatory and the Digital Legal Pack can still be completed and signed.
By making clients aware of what documents they need to provide in order to sell their property, it means they can source these earlier in their sale (i.e. during marketing) to prevent their transaction being slowed down... because by the time a buyer has been found the conveyancer handling the sale will need the documents. If these documents still can't be found by the time the Digital Legal Pack is being shared with a conveyancer, then that's ok too and the Digital Legal Pack can still be shared.
If at a later stage these documents are sourced, then they can be retrospectively added to the Digital Legal Pack but the client will also need to let both estate agent and conveyancer know so that they are aware and can download them if necessary.
Once a Digital Legal Pack is shared with a conveyancer, then if a client tries to make a change they are notified on screen that they can do so but that they will need to let the estate agent and conveyancer know.