This is a bug in the compiler (linker) used by the Arduino AVR Core. It's not because of a bug in your code, and there's not much you can do about it except using a different version of the AVR Core.
See this forum post:
Here's the traditional workaround:
Tools > Board > Boards Manager
Wait for downloads to finish.
When you move the mouse ...
strcat is a function to concatenate strings. It expects a char array as second parameter. You have only a single char newChar to add to your string.
size_t l = strlen(inputs[i]);
inputs[i][l] = newChar;
inputs[i][l + 1] = 0;
Note: It would be better to track the length of the inputs then use strlen. strlen iterates over the array until terminating ...