Open Trash from the Dock and note the item named in the Finder message. If it is in use, save your work and quit the app that has it open. If it is locked, select it, open File → Get Info and clear Locked if you are authorized to do so. Then try deleting that item by itself.
01If Finder says the item is in use
Section linkClose the document and quit the app that was using it. Closing a window can leave the app running, so use the app’s Quit command. If the item is an app, make sure the app itself has quit. If the warning persists, save open work and restart the Mac before trying again.
02If the item is locked or permissions block you
Section linkSelect the named item in Trash and press Command-I for Get Info. If Locked is checked, clear it for a file you are allowed to change. If Finder reports a permissions problem, check Sharing & Permissions in the same window. For a shared or managed Mac, ask the administrator about the named item before changing access.
03Delete the intended item by itself
Section linkOpen Trash, Control-click the item and choose Delete Immediately. Read the confirmation before selecting Delete. This permanently deletes the selected item. If you might need a file again, choose File → Put Back before emptying the Trash.
04If the same item still blocks you
Section linkRecord the item name, the exact Finder message, and whether the item came from the internal disk, an external drive, or iCloud Drive. If the problem is limited to one drive or shared folder, give those details to Apple Support or the person who manages that storage.
Sources & further reading
Put it into practice
Before you convert
A short checklist for choosing an image format and checking the result.
Download checklist (.txt)