[rk3568][Ubuntu18.04]fail to find distribution for tensorflow through pip

ruby2018

Issue: fail to find distribution for tensorflow through python/pip

Hardware: rk3568 dev kit from firefly
OS:            Ubuntu 18.04
                  (downloaded from firefly: http://en.t-firefly.com/doc/download/page/id/89.html)
Python:      3.6.9
pip：          21.1.2 from /home/firefly/.local/lib/python3.6/site-packages/pip (python 3.6)

Use pip to install tensorflow in linux terminal, but the result shows that there is no distribution found:  
(1) firefly@firefly:~$ pip install tensorflow
Defaulting to user installation because normal site-packages is not writeable
ERROR: Could not find a version that satisfies the requirement tensorflow (from versions: none)
ERROR: No matching distribution found for tensorflow

(2) firefly@firefly:~$ python3 -m pip install tensorflow
Defaulting to user installation because normal site-packages is not writeable
ERROR: Could not find a version that satisfies the requirement tensorflow (from versions: none)
ERROR: No matching distribution found for tensorflow

Does someone have a clue what the cause for this issue could be?