If you're attempting to integrate with Kaseya BMS and are receiving an Invalid Credentials error, provided the credentials are valid, the next step in troubleshooting will be to confirm the Kaseya BMS User has API Access.
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The first step will be to determine the Security Role assigned to the User. This can be checked by navigating in Kaseya to Admin > HR > Employees. Once there, click on the Open/Edit icon on the far left column to view the User. Clicking on the 'Security Roles' tab will show a checkbox under the 'Has Role' column to confirm the assigned Security Role:

The second step will be to click on Admin > Security > Roles clicking on Open Edit Icon in the far left column:

Click into the accordion for 'Admin' and scroll down under Special Features to see if 'Has API Access' is checked:

If unchecked, please check this box and save the Security Role. If you're unable to access this section, please get in touch with the administrator of the account to make the change or setup the integration for the account with the required API Access.