Using the Finder can you create a new folder at the same location you are trying to save to in Excel?
If you cannot create a new folder using the Finder, then make sure you have a good system backup immediately. After you have a system backup, try rebooting your laptop.
Can you select the "Save As..." option in Excel to save the file with a different name and make a new copy of the file (you may need to hold the Option-click the File menu in Excel to see the "Save As...." option).
Do you have any free space left on your drive?
Are you running any anti-virus or third party security or cleaning apps which may be interfering? Run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper.