Spotlight will not stop reindexing.

Is there are error message beyond saying the operation can not be completed?

Suggest downloading the Application Etrecheck directly from a well Respected ASC Contributor. 

The application is free or paid from added features. 

Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ).

It will take a Snap Shot -  both the hardware and software.

 The Report will Not Reveal Any Personal Information. 

Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

This includes AntiVirus, Disk Cleaners, Disk Optimizes, UnInstaller etc.

The The Built in Security  is all that is required.

Anything that appears on this list needs immediate attention.

Battery failure - Your battery is reporting that it needs to be serviced.

A - Find an Apple Authorized Service Provider and have the computer evaluated

SSD too slow - SSD is showing poor performance.

A - Find an Apple Authorized Service Provider and have the computer evaluated

More than one antivirus app - This computer has multiple antivirus apps installed. Antivirus software: Apple, CleanMyMac, and Trend Micro

A - CleanMyMac - read the first 600 posting related to this Application and Remove it as per the Developers Instructions.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

Trend Micro Antivirus

A - There are no known Windows-like Viruses in the wild that self replicate and affect macOS, because of the underling UNIX  Foundation and Permission Limitation. The only thing this software is Protecting is the Bank Account of the Developers of this software.

Same thing read the first 600 posting on the ASC Community. Remove it as per the Developers Instructions

The The Built in Security  is all that is required to protect the computer.

System modifications - There are a large number of system modifications running in the background.

Runaway user process - A user process is using a large percentage of your CPU.

x86-only Apps - This computer has x86-only apps might not work on future versions of the operating system.

Kernel extensions present - This computer has kernel extensions that may not work in the future.

Rebuild the Spotlight index on your Mac

Still waiting for the Etrecheck report to see what hardware and software is involved on this computer.

The Application has been reported to be used by Apple Support themselves to assist in such issues.