Currently, Microsoft Teams is not connected to the Government Tenant so it won't pull in data from there like it's supposed to. Maybe some day in the future it will be.
However, there is a workaround solution to accomplish this goal per below:
1. Go to https://app.powerbigov.us
2. Go to the report or dashboard in Power BI, then click on share and copy the URL link to it
3. In Teams, add a tab + and click on the Website icon
4. Type in a Tab name
5. Paste the Power BI to the URL* you copied from Step 2 above in the link box then click on the Save button