The compilation fails because the Time.h file that the compiler finds has nothing to do with time libraries such as that by Paul Stoffregen (https://github.com/PaulStoffregen/Time).

I tried your Sketch, compiled for an Arduino Uno, and saw the same error you see: that Time.h resolves (the file exists somewhere), yet now() is not defined by that Time.h

After searching my Windows PC for a while, I finally found what I think is the file that #include includes on my installation: C:\Users\Brad\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.2\firmwares\wifishield\wifiHD\src\time.h or perhaps C:\Users\Brad\AppData\Local\Arduino15\packages\arduino\tools\avr-gcc\7.3.0-atmel3.6.1-arduino5\avr\include\time.h

Neither of those files defines the now() function.

If you want to use Paul Stoffregen's Time library, download and install it from https://github.com/PaulStoffregen/Time. If instead you wish to use Michael Margolis' Time library, you can find and install it in the Arduino IDE, under Tools / Manage Libraries... and entering "Time" (without quotes) in the search term.

As others have pointed out, the Arduino environment doesn't always know the current date and time. The functions mills() and micros(), return the number of milliseconds or microseconds, respectively, since the Arduino booted. For just looking at the passage of time, most people use millis() or micros() instead of a more complex library.