The alias “...” can’t be opened because the original item can’t be found.

An alias is a link to the actual item. The message is saying that the linked item can't be found.

What type of files are in the folder?

In Finder can you see the files in that folder?

What is size of the folder, click on the folder and select Get Info

If you select one file and what does Get Info show? Posting a screenshot of the Get Info window would be great.

To take screenshot

Take a screenshot on your Mac - Apple Support

To add it to a reply

Click on the add image icon right below the test entry box

fter clicking on the icon you are present with a Finder like windows. maneuver to find the image file you want to post and click on it and select Open in bottom right or double click on the image file to add the image in the text reply box.

I would first remove Avast and CleanMyMac according to the developer's instructions. Then rerun Etrecheck to see if there are any leftover Avast or CMM files listed. If any are left boot into Safe Mode according to How to use safe mode on your Mac and and remove them manually.

Once completed, boot normally and run for a while to see if the performance is back to normal.

Also make sure you get a couple of new backups into Time Machine so if you do need to restore you won't be bringing back those apps or their files.

What happens if you now add a file to that folder? Can you still read it later?

Select one of the"Alias" file and enter the Shift and R. What options are displayed?

When did this problem start with respect to what you did?

Any power failures or disconnections of the disk?

Like upgrade macOS version, added files to this folder.

Is this aHDD vice SSD?

What manufacturer and model ?

How old?

Have you tried repairing/First Aid the disk?

Have you used any "cleaning" apps or other non-Apple disk repair apps on the disk?

My guess is that an application deleted the source folder. Hopefully, you will be able to restore it from Time Machine.

There is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is desshoigned solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Also, unless you're using a true VPN tunnel, such as between you and your employer, school or bank's servers, they are useless from a privacy standpoint.  Read these two articles: Public VPN's are anything but private and Former Malware Distributor Kape Technologies Now Owns ExpressVPN, CyberGhost, Private Internet Access, Zenmate, and a Collection of VPN “Review” Websites. 

Yes, you would be best served by deleting all of those A/V and similar types of apps according to the developer's instructions.

The Screenshot app let's you capture a Finder (or any other) window as an image.