I am using linux ubuntuDDE 20.04, Freecad appimage version 22411(Git).

From freecad:
"Se han encontrado algunos errores que impiden instalar este entorno de trabajo: Falta módulo python: PyFoam, . Por favor, instale los componentes que faltan primero."

I translate from spanish:

Some errors have been found that prevent install this workbench, Missed module python:PyFoam, Please install the missed components first.

From O.S. terminal:
:~$ sudo pip3 install PyFoam

Requirement already satisfied: PyFoam in /usr/local/lib/python3.8/dist-packages/PyFoam-2020.5-py3.8.egg (2020.5)
Requirement already satisfied: numpy in /usr/local/lib/python3.8/dist-packages/numpy-1.19.2-py3.8-linux-x86_64.egg (from PyFoam) (1.19.2)

Anyone knows how to fix it and want to share it? : )