Installing sage 9.5 from source on Ubuntu 20.04

edit

After running the command CC=/usr/bin/gcc-7 CXX=/usr/bin/g++ FC=/usr/bin/gfortran-7 ./configure --prefix=$CONDA_PREFIX

I get the following error

configure: error: 

Given --with-system-gcc=force, but no system package could be used.
That's an error.  Please install the indicated package to continue.
(To override this error, use ./configure --without-system-gcc)

What could be the possible reasons and solution for this.

Thanks