Grantt Chart is not visible in the Planning

Hi,
Under the Planning, only Timeline is visible. The grantt Chart, table  is not visible. If the Timeline option is also disabled then the screen is empty. I verified the settings of the visibility of the required fields like Id, Name, Duration, Start, End, etc are set. Even then nothing is visible. Looks like some settings in the parameters is causing the problem. But I could not locate the right parameter and even I dont now whether the parameter is cause of this problem. 

Kindly help. 

Hello,

Below is the Error message:
2025-04-18 21:37:56.212 ***** ERROR ***** [V12.1.0] [4] ERROR *****
2025-04-18 21:37:56.213 ***** ERROR ***** [V12.1.0] [4] on file '/var/www/puvys.pm.in/tool/jsonPlanning.php' at line (991)
2025-04-18 21:37:56.213 ***** ERROR ***** [V12.1.0] [4] cause = Undefined variable $ref

Now I vaguely remember, couple of days before under report I selected json export of data. Now I am not getting which report. Below is the screenshot:
 

I am using version 12.1.0