Hope you are doing well and sorry for the trouble. As described, you are unable to select the text using mouse.
Would you mind sharing the steps you are doing to select the text in the PDF by moving mouse? Also, make sure you have select the selection tool as shown in the screenshot.
If possible, please share the screen recording of the issue for a better understanding.
What is the version of the Acrobat Reader you are using? To check the version go to Help > About Acrobat and make sure you have the latest version 20.12.20048 installed, from the help menu > Check for updates.
If it doesn't work, please repair the installation (Windows Only) and check. Go to Help > Repair Installation and reboot the computer once.
If you are using the Hand tool for text selection, please try the following preference settings. Go to Edit > Preferences > General > Check 'Make hand tool select image and text' > Click OK and reboot the application.