Your code is currently trying to read messages, by using a GET HTTP request, which is the .get() part in your code.
If sending a message is what you want to do, then you need to make a POST request instead, by changing the .get() to .post(channelMessage).
More on the Protected APIs
According to the "Protected APIs" documentation:
Microsoft Teams APIs in Microsoft Graph that access sensitive data
are considered protected APIs. These APIs require that you have
additional validation, beyond permissions and consent, before you can
use them.
Personal messages appears to be sensitive data, which makes sense. However, note that there is an alternative solution where you use the API as if the user was using the API. It is called Delegated permissions.
Delegated permissions are used by apps that have a signed-in user
present. For these apps, either the user or an administrator consents
to the permissions that the app requests and the app can act as the
signed-in user when making calls to Microsoft Graph. Some delegated
permissions can be consented by non-administrative users, but some
higher-privileged permissions require administrator consent.
If delegated permissions isn't an option for you, you can still make a formal request to Microsoft Graph, asking for permissions to use the API without any user interaction.
To request access to these protected APIs, complete the following
request form. We review access requests every Wednesday and deploy
approvals every Friday, except during major holiday weeks in the U.S.
Submissions during those weeks will be processed the following
non-holiday week.
Here is the form for sending a formal request.