Hi Ghada,
Thanks for sharing your experience with us.
For the specific folder that cannot sync properly, you may open the folder and check which file has the "red x", it should be the file that causes the issue. Move the file from the folder and see if the rest files and folders will sync normally.
If you modified a file in the folder from local sync folder and then the issue happens,
you may try following:
1.Stop sync the SharePoint library by right click OneDrive icon in task bar>Settings>Account> find the document library and click
Stop Sync.
2.Move the affected folder to a different location on your computer, such as Desktop.
3.Then go to SharePoint online, click the Sync button to sync the document library again.
4.After the sync complete, put back the modified file in the sync folder.
If issue persist, please share with us following information for troubleshooting:
1.The version of your OneDrive sync client.
2.If you open the affected folder, can you find a file that has the red x too?
Can you get any detailed sync problem for it? If yes, please share a screenshot about it.
Best Regards,
Tina