How to fix: Invalid conversion from 'const char*' to 'char*' [-fpermissive]

This begin() method expects a modifiable character array as its first argument. That's what you should provide:

char ssid[]           = "YOUR_SSID";      // this is changed
const char* password  = "YOUR_PASSWORD";  // this is fine
[...]
WiFi.begin(ssid, password);

The location where you are calling the function begin, has as first parameter a parameter of type const char* instead of char* ... remove the const from this argument type.

Probably you have something like

const char* s = ....

...

...begin(s, ...)

Change class s to

char* s = ...