While authenticating your Vtiger account on Power BI, please paste the Access Key found in the My Preferences page of your Vtiger account and not the Login password.
You can find the Access Key by following these steps:
1. Click on your user icon
on the right top
2. Click on My Preferences
3. Copy the Access Key
4. Paste it while authenticating Vtiger in Power BI
Refer : Power BI