We are sorry for the trouble. As described you the Read out loud is not working properly.
Is this a behavior with a particular PDF file or with all the PDFs? Please try with a different file and check.
Would you mind sharing the version of the Acrobat Reader DC you are using? To check the version go to Help > About Acrobat and make sure you have the latest version 20.12.20043 installed. Go to Help > Check for Updates
Also, please try to repair the installation (Windows Only) Go to Help > Repair Installation
Are you on a Mac Or Windows machine?
For testing, please try to turn off the protected mode (Windows Only) and see if that make any difference. Go to Edit > Preferences > Enhanced Security > Uncheck 'Enable protected mode at startup', turn off the protected mode and uncheck the enhanced security > Click OK and Reboot the application and check.
Note: Turning off the security may possess security risks, please turn on the security after testing.