If you want reviewers to allow to select their preferred papers, please use the bidding function of ConfTool Pro.
Please go to:
Overview => Settings => Conference Phases and Deadlines
... and enable the corresponding phase "Bidding: Reviewers Can Bid for Submitted Contributions".
For the option "Enable Module and Select Start and End Date" move the toggle switch to "Enabled".
Continue to define the phase via the option "Set Start and End Date".
Furthermore, make sure that the bidding function is enabled for each of the submission types / tracks (it is enabled by default).
Please go to:
Overview => Settings => Manage Submission Types / Conference Tracks
... and edit all relevant submission types / tracks.
Ascertain that in the section "Review Options" the option "Bid for Contributions" is enabled (this is the default, see Image 1).