Same story here...
Elementary OS 5.1, i cannot achieve any sort of installation.
$ sudo apt install elementary-tweaks
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package elementary-tweaks"
AppCenter is attempting to check for Operating Software Updates.
The solution is to update/correct the source lists included with the distribution.
Replace all instances of "hera" with "bionic" (in 5.1 Hera) in the following 4 files: