Hi MarianoCampo1980,
I apologize for all the trouble it has caused and sorry about your time.
As per your description, I just tested the Excel
16.44 (20121301)
version, and it seems to work fine from my side on Mac OS Big Sur. I
click anywhere in the PivotTable to show the Design tabs and I’m able to see the features.
See image below for your reference:

To move further, please try the following steps and see if it works for you.
1.Remove Excel cache and check if issue happens:
Close Word app > right click Finder from dock and select Go to Folder >
enter ~/Library/Containers > move the folder Microsoft Excelto your Desktop > reopen Excel, create a new document, type texts in it and
check the result.
2.Try to reboot Mac in safe mode and determine whether background programs are causing the issue. For detailed information, please check this
article.
Disclaimer: Microsoft provides no assurances and/or warranties, implied or otherwise, and is not responsible for the information you receive from the third-party linked sites or any support related to technology.
If above suggestion does not work, to narrow down your issue, could you please confirm with us your Mac OS version and please confirm does issue happen to specific Excel file on your mac.
I appreciate your patience and understanding.
Best Regards
Waqas Muhammad