Hello,
I have one joomla site version 3.9.24 and mobile pro component is installed of version 2.1.48 .
I am getting error below mobile.
Fatal error: Method HTML_Node::__toString() must not throw an exception, caught Error: Class 'Ressio_Cache_File' not found in /home/xxx/public_html/administrator/components/com_mobilejoomla/ress/classes/htmloptimizer/pharse.php on line 0

plz tell me how to fix it.

Could you check that file administrator/components/com_mobilejoomla/ress/classes/cache/file.php exists? If it isn't, try to reinstall the Mobile Pro Joomla extension.
Otherwise, what PHP version do you use (I'll try to reproduce the issue)?

yes...that file was missing...i added that and issue is fixed.
thanks a lot !!!

Check other files exist too. I'd recommend to install Mobile Pro over existing (i.e. currently installed) version, it will keep your settings, but recreate all the files.