Ubuntu 20.04 is not a supported OS for MongoDB 4.2 or earlier (meaning any current version of MongoDB at this time).
@Stennie_X mentions in another thread that support for Ubuntu 20.04 is planned for MongoDB 4.4.
Unfortunately this means that you will have to wait until the 4.4 version of Mongo is released unfortunately. If this is a playground/test system, you might be able to pull down the zip file from the MongoDB downloads page and use it that way.