Sensitivity labels not showing up in document library

Can you confirm what you are not seeing?

Did you want to add a default sensitivity label to a document library Add a sensitivity label to SharePoint document library - Microsoft Support? If so, this is an advanced feature, so you need one of the A5/E5/G5 SKUs.

If you have the necessary licenses, please note that not all labels can be selected. Here are the limitations

• Let users assign permissions when they apply the label and the checkbox In Word, PowerPoint, and Excel, prompt users to specify permissions is selected. This setting is sometimes referred to as "user-defined permissions".
• User access to content expires is set to a value other than Never.
• Double Key Encryption is selected.

See Configure a default sensitivity label for a SharePoint document library | Microsoft Learn

If you cannot add a sensitivity label to a file in a Document Library, then check that you have enabled the functionality Enable sensitivity labels for files in SharePoint and OneDrive | Microsoft Learn

If you do not see the sensitivity label in the library view, then this is hidden by default. Select +Add column, then Show or hide columns and then tick the Sensitivity column and finally Apply