Currently it is not possible to create aliquots of type different than that of the parent specimen.
There needs to be a derivative of the type frozen or fixed tissue and then create aliquots out of the derivative. The hierarchy would be like below
Parent specimen - Fresh Tissue
Derivative - Frozen Tissue
Aliquot 1 - Frozen Tissue
Aliquot 2 - Frozen Tissue
However, in the latest version it is possible to create an aliquot directly from the parent specimen and OpenSpecimen will create an intermediate derivative.