You are using python 3.8, which was not officially supported when tensorflow was at version 1.15. You can also check on pypi, there are no files available for cp38, even for 2.10 Onle the versions listed by your command have a cp38 whl file available, see here
Since you have conda, simply create a virtual env with the required version
conda create -n tf python=3.7
then install tensorflow in this env