I have a 13 inch MacBook Pro 2015 model. The Mac had Big Sur installed on the internal SSD. This internal SSD died and thanks to macOS Recovery over the internet (Option-Command-R), I managed to install El Captain on an external SSD drive.
I am trying to upgrade to Big Sur, but the option to choose my external SSD during installation is greyed out with the message
"You may not install to this volume because the computer is missing a firmware partition".
Is it possible to install any macOS higher than Catalina on an external SSD?
Below is the output from the command