Hello Venessa,
Thank you for your response.
Could you please elaborate on your first question that which kind of extension or how I can find any extensions are affecting existing functionalities.?
And for your second question, I didn't run Budget matrix directly I followed your steps to create Budget so I first visited Budget Page (113) and created Budget 2020 and then went for matrix (9203) page. But I found this as given below, please see highlighted message.
And I have checked my permission sets, I have one permission set which is D365 FULL ACCESS which gives me full access to table data. 113 and 9203 are object IDs for 'page' object type right? Could you please tell me what object IDs are there for 'table data' object type? So Once again I will verify in permission sets for these tables.
And for me, there is only one user available in my users list table of BC so I can't check for other users.
One more thing, when I change show as lines to "Business Unit" instead of G/L account. It will show months for year in view.

Then why it is not showing for "G/L Account" in show as lines. Is there any need to set/change something in available G/L account card page like some enable options or something field's value?