Connecting to model database doesn't work

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Hi,

I am trying to follow this tutorial but the models present in the postoffice.world file do not load.

I realized that this might be a problem with Gazebo itself because, as you can see in the image below, it doesn't connect to the model database.

I looked around the internet but I can't find any recent thread on this issue and the old solutions don't work.

A suggested solution here recommends to clone the repository that contains all the models, however I don't understand where I should clone it and how to then link it to the .world file.

Any suggestions?