
Please check the following points to troubleshoot this issue:
1.Click Library Settings >> Advanced Settings >> in the List experience, select New experience option
2.Have you use some features that can only be applied in Classic experience?
For example: create External data column, use multiple web parts on List view page.
For more restrictions, please refer to the article Differences between modern and classic experiences for lists and libraries
3.Check "All documents " view of other Document Libraries can support switch SharePoint mode.
Thanks
Echo Du
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