Bought mailwizz and got Error establishing a database connection while setting Email campaign

Bought mailwizz and successfully set up mailwhizz on my domain as shown in these kb articles
INSTALL STEPS: https://kb.mailwizz.com/articles/install-steps/
(Follow these only if you install a fresh copy of the app)

__

GETTING STARTED STEPS: https://kb.mailwizz.com/article-categories/getting-started/
(Follow these only if you are new to using the app)

I successfully created Customer ,did delivery settings,added subscribers to customer ,But now got Error establishing a database connection while setting Email campaign

please let me know how to remove this error of database connection error while setting up email campaign

Tarun

Contact author directly via their support board