Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘add_custom_scripts’ not found or invalid function name

add_action takes a valid callable function as second parameter. WordPress is unable to find add_custom_scripts function which is being used to add some custom scripts probably, may be its not defined anywhere. You might have to look into fucntions.php file of your active theme/child-theme, find the add_action where second parameter is passed add_custom_scripts. There are two things you can do after finding this line.

  1. Change the add_custom_scripts to a valid callable function.
  2. Comment that line if you don’t need want to use that .

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