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Just to confirm at the beginning, roslaunch is installed. The first time I saw the build error below I installed it with sudo apt install python3-roslaunch. If that method of install is the problem that'd be an easy fix, but I think it's okay.

No, it's not okay, and that is most likely the cause of your problems.

python3-roslaunch is not the correct package, but an UpstreamPackage. The correct package would be ros-$ROS_DISTRO-roslaunch. In your case: ros-noetic-roslaunch.

You cannot (and should not) mix these two package sources.

See the linked wiki page for more information, and see “upstream packages” increasingly becoming a problem on ROS Discourse for a meta-discussion about them.

PS: I am surprised though how you could have a ROS Noetic installation without roslaunch, as it's a very low-level dependency.

Seeing as you've already installed (s)one upstream package(s): could you tell us how you've installed ROS? Perhaps you have only a partial install.

Just to confirm at the beginning, roslaunch is installed. The first time I saw the build error below I installed it with sudo apt install python3-roslaunch. If that method of install is the problem that'd be an easy fix, but I think it's okay.

No, it's not okay, and that is most likely this could be the cause of your problems.

python3-roslaunch is not the correct package, but an UpstreamPackage. The correct package would be ros-$ROS_DISTRO-roslaunch. In your case: ros-noetic-roslaunch.

You cannot (and should not) mix these two package sources.

See the linked wiki page for more information, and see “upstream packages” increasingly becoming a problem on ROS Discourse for a meta-discussion about them.

PS: I am surprised though how you could have a ROS Noetic installation without roslaunch, as it's a very low-level dependency.

Seeing as you've already installed (s)one upstream package(s): could you tell us how you've installed ROS? Perhaps you have only a partial install.