Edit own format in Number not possible

Are you sharing this Numbers file with others?

If Yes, then here the explanation why it is not working.

Collaborate on documents in Pages, Numbers, and Keynote on iPhone, iPad, and Mac - Apple Support

Features not supported with collaboration

If you need to use one of these features, stop sharing the document, make your changes, then share the document again. When you stop sharing, the document is removed from iCloud Drive for all participants. If you've restricted the document to "Only people you invite," you will need to re-invite everyone after.

These features aren't currently available when you collaborate with other participants:

Pages, Numbers, and Keynote

• Reduce File Size (File > Reduce File Size).*
• Drag rows and columns between body and header areas in tables.
• Create, delete, or reorder styles.
• Edit with Ruby in Asian languages.
• Create or edit custom cell formats.
• Replace every instance of a font in a document.

Based on the screenshot it looks like you speak German.

Please read this post

Eigenes Datenformat in Numbers - Apple Community

Ralf

What do you mean by "own format"?

SG