Deepak

Should check few things

A) using result returned from mysqli_query (which is the result set ) as a string.

for example:

$resultSet = mysqli_query($mysqli, “SELECT * FROM table”);

echo “Employee:”. $resultSet;

Therefore code here “Employee:”. $resultSet; will complain as “mysqli_result could not be converted to the string” because we are using resultSet as a string

Can find here more examples for the root causes. could not be converted to the string
https://www.drupal.org/forum/support/post-installation/2010-12-22/recoverable-fatal-error-object-of-class-mysqli_result

B) Make sure the latest PHP and WordPress versions are installed.

You can try this way too as mentioned on stackoverflow.

https://stackoverflow.com/questions/15238288/cant-login-to-my-wp-admin-after-switching-to-ssl