I'm using Buster Lite and ran into a problem with installing usbmount. The error I got was "unable to locate package usbmount". I found the solution, and I am append my solution to this thread since this thread shows up on some search-engines pretty high up.
The solution I used was to clone the github repo from https://github.com/rbrito/usbmount/ and build it (instructions below is partially from the readme)
git clone https://github.com/rbrito/usbmount.git
cd usbmount
sudo apt-get update && sudo apt-get install -y debhelper build-essential exfat-fuse exfat-utils
sudo dpkg-buildpackage -us -uc -b
cd ..
sudo dpkg -i usbmount_0.0.24_all.deb
Additionally, use
sudo nano /etc/usbmount/usbmount.conf
to add exfat to the list of file-systems
save and reboot