EDIT:
With the added info you posted, it seems you added backports ppa during the ubuntustudio-installer
run to stay current with the latest packages.
If you are ok without that (at least to begin with, and it is most likely enough), try removing that ppa, and installing again.
In the worst case scenario, there is a mismatch in packages/versions between what is available among the different repos. Further tinkering could clarify this.
This is certainly strange, as I could not locate package studio-controls
in my 20.04.1 apt-cache
:
$ apt-cache depends ubuntustudio-installer
ubuntustudio-installer
Depends: tcl
tcl:i386
Depends: tk
tk:i386
Depends: debconf
Depends: policykit-1
policykit-1:i386
Depends: ubuntustudio-controls
Depends: software-properties-common
$ apt-cache depends ubuntustudio-controls
ubuntustudio-controls
Depends: a2jmidid
Depends: jackd2
Depends: pulseaudio-module-jack
Depends: python3-dbus
Depends: python3-gi
Depends: python3
Depends: python3-jack-client
Depends: zita-ajbridge
Depends: qasmixer
Depends: pavucontrol
Conflicts: <cadence>
Suggests: carla
$ apt-cache search controls | grep studio
ubuntustudio-controls - Ubuntu Studio Controls
As per this, it appears studio-controls
it is available only for groovy (20.10) and hirsute (21.04).
Please post the version of packages and other info via the verbatim output of
$ dpkg -l | grep ubuntu | grep studio
$ dpkg -l | grep controls | grep studio
$ apt-cache policy ubuntustudio-controls studio-controls
$ uname -a
$ apt-cache depends ubuntustudio-installer
$ apt-cache depends ubuntustudio-controls
$ apt-cache search controls | grep studio
Did you add any ppa?