The wp_cache_get() function is part of wp-includes/cache.php file.
Check that cache.php exists
Please make sure that you have a cache.php file inside your wp-includes/ folder. File: wp-includes/cache.php
Next step?
If the file does not exist, your installation of WordPress may be corrupt. You’ll need to upload the missing files even manually by first downloading the latest WordPress version and ftping the missing files.
If the cache.php file exists
Download it and open it with any text editor on your device. Then please look for this function
wp_cache_get()
If you can’t find this function, then please proceed with previous step.
If you can find this function, then somehow the function is blocked, in this case you’ll have to contact your webhost or system administrator to get it sorted.
To read more about this function, where it is placed, and what does it do please refer to the codex here:
https://developer.wordpress.org/reference/functions/wp_cache_get/