fillna() in python node is not working as usual

I am trying to deploy an existing python code into KNIME but I faced some issues while simply trying to change the type of a column (which had missing values). Normally this is not a hinderance in python.

Here is the data type as per df.info()

Upon trying to use fillna() i got this error:

So I tried to use Missing Value Node of Knime’s but I am unable to use “Fixed Value”

My trials with Multi column string manipulation did not work as well.

Any solution to my above problems will be very helpful