Hi @sns ,
1)First of all, please check if you have enabled Quick Edit in list settings> Advanced settings:
2)If the list view has been "Group by" enabled and this will disable the “Quick Edit"option, check it and remove the Group By:
3)Go to the "Tabular View" option under "Modify view" and make sure that the "Allow individual item check-box" is checked.
More information for your reference:
https://vigneshsharepointthoughts.com/2015/10/14/quick-edit-option-not-working-on-sharepoint-list/
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