About Quick Edit button
Quick Edit button is an another alternative option replacing datasheet view in modern SharePoint Online pages. In some scenarios, Quick edit button does not appear as expected. There could be many reasons, please follow below troubleshooting steps to resolve this issue.
1. Make sure "Allow items in this list to be edited using Quick Edit?" option is selected.
Go to List settings > Advanced Settings > Scroll down to "Allow items in this list to be edited using Quick Edit?" and select Yes.
2. Check if you have Edit permission on list level. Even if edit permission granted on Item level, you still need edit permission on list level to see the "Quick Edit" button.
3. Try using default list view without using options or filters like "Group By" or "Filter" or "Sort" etc.
4. Although "Quick Edit" button works fine on all browsers but try resetting your browser history / cookies or using browser in private mode to see if it work fine.
5. If list with datasheet view recently migrated from on-premise SharePoint to SharePoint online, then after migration try deleting the datasheet view and recreating new one