Sorry for the delay in response.
I assume you are trying the conversion through the desktop application Adobe Reader DC? If yes, then do you get the Sign In error when you select a pdf and try exporting it using the "Export PDF" tool?
You may check the following-
1- As it gives you Sign In error, so before starting a conversion you may Sign In to the application first. Make sure that you see your name at the upper right corner of the app.
2- Check is you are able to complete the conversion online, via- https://cloud.acrobat.com/exportpdf
3- If it works online, then you may check the desktop application's update. Open Adobe Reader>Help>Check for Update.
4- If there is any internet security software, then please turn it off for a while and try exporting the file.
[Antivirus software can help protect your computer against viruses and other security threats. In most cases, you shouldn't disable your antivirus software. If you have to temporarily disable it to install or activate other software, you should re-enable it as soon as you're done.]
Let us know the update.