Get result of Python function in Labview and reuse as value for next function
LabVIEW specifically calls out that the only options allowed for passing data to/from Python Nodes are:

Numerics
Arrays, including multi-dimensional arrays
Strings
Clusters

Given this list of options I would suggest passing your data back/forth to LabVIEW as strings using the option provided by:

DataFrame.to_json
pandas.read_json

With the addition of a few lines of code to the python segment you get your data frame out into LabVIEW in a human readable and parsable datatype.
I try to create a dataframe in a Python function used by Labview. In each LabView loop, I want to concat a DataFrame in a Python function. How can I reuse the output of the last function for the next Python function using Dataframe as output? When I connect the function output, I get the error in LabView: "Polymorphic terminal cannot accept this datatype".
Other DataTypes (e.g. array) are not a problem.