What will you do if the developer cannot see the bug?

First, I will try to obtain as detailed logs and environment information as possible (OS version, browser, network settings, etc.).

If the logs do not clarify the situation, I will ask the developer to step through the process of reproducing the error on their local machine or test server, possibly using screen sharing. This will help identify differences in configuration or sequence of actions.

I can also provide test data that closely resembles "production" conditions to exclude the influence of incorrect input data.

If the error is specific to a certain environment, I will try to reproduce it on the developer's machine or environment.

If all of the above does not help, I will suggest pair programming/debugging with the developer to jointly investigate the problematic code section and process that leads to the failure.