As @AsmaZinneeraJabir mentioned, this code compiles with ballerinax/slack:3.3.0.

The way to make it work is to :

• Delete Dependencies.toml
• Build the project (bal build)
• Edit Dependencies.toml by putting v3.3.0 for ballerinax/slack (line 326)

Another way for the training 1 project to compile is by changing this code to fit the latest version of ballerinax/slack specs (which obviously works). But it is better to stay close to the initial code.

Thanks again, My initial question is answered