return inData;
// Reset for the next packet
started = false;
ended = false;
index = 0;
inData[index] = '\0';
Several problems here. First, nothing after the return statement will be executed. Second, inData is a char array, not a String, so the function return type does not match what you are trying to return.
but what about my library, do you know how to get it to work?
Where is your sketch?
ISO C++ forbids declaration of 'Testing' with no type
I don't see a declaration of anything called Testing, with or without a type. Post ALL of the errors, and ALL the code being compiled when the errors are generated.