Hi,
What version of Office are you using now? Do you use Office 365 for Mac or Office 2019 for Mac?
I test in Office 2019 for Mac, Version 16.30. When there is a table in Excel, the Custom Views will be greyed out as the following picture.

It is an update behavior, here is a similar thread you may have a look: Suddenly Custom
Views is dimmed in Excel 365.
When you have suggestions for this update behavior, to let the developers hear you, I would suggest you raise the question into
Excel for Mac UserVoice forum.
Besides, you may click the smile icon > Tell Us What Can Be Better, then you could submit a feedback on this update behavior.

Hope the information could help you.
Best Regards,
Emily
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