The requirement for the completion of penance after confession is that it should be done and should be done soon. There is no restriction on receiving Communion while the penance is pending. The absolution is efficacious so long as the penance is completed afterwards.
However, if the penance is not subsequently performed, especially when it is due to neglect or deliberate avoidance of the penitent, not only may it incur a grave sin against the Eucharist, but it would invalidate the confession. One must confess this failure, as well as re-confess the sins from the invalid confession.