Completely reinstalled the TQSL program and downloaded my .tq6 certificate file from the lotw web site.
I don't think it works that way. From the ARRL website:
Certificate or Private Key Not Found
When a certificate request is made a TQ5 file is created that you send to LoTW. In response a TQ6 file is sent back to you. These files contain unique keys therefore the TQ6 file must match the TQ5 which made the request.
This message occurs when a TQ6 file cannot find the matching file which made the certificate request.
Some causes for this error message are that the TQ5 file was deleted before the TQ6 file was loaded, you are attempting to load the TQ6 file into a computer that did not make the original TQ5 request, you made a new certificate request before the first request was processed.
If you receive this message click OK and continue loading the certificate file. If you do not acheive a gold ribbon certificate then request a new certificate.
The simplest solution to most certificate issues is to request a new certificate.
The best way to avoid this type of problem is to create a backup of your certificate using tqslcert.exe. Highlight your certificate then save. This will create a p12 file which can be imported or moved to another computer.
Unfortunately in your situation I think you're going to have to request a new certificate.
73, Alan N5NA