Play Back Volum

After recording a cassette, tryed to play it back, volume was very low,

If you go to Effect → Amplify and set the New Peak Amplitude to 0dB, that will normalize (maximize) the volume. (0dBFS is 100%, or 1.0, or essentially as loud as you can go digitally without clipping/distorting the peaks, or without using dynamic compression.)

When you look at the waveform in Audacity, how high does it go on the 0-to-1 scale? If the peaks go up to around 0.5 or more, that’s just about perfect and you simply need to Amplify.

Are you connected to the line-level RCA jacks on a cassette deck? And, are you connected to the line-in on your soundcard? Is this a homemade recording, that might have a lower-level, or is it a commercially-produced cassette?

If the original recording level is way too low, you can get noise problems, because when you increase the level you also increase the any background noise added/picked-up during recording. This might not be a problem, since the noise on a cassette is usually worse than any noise added by your soundcard, and the noise present on the cassette stays in proportion to the audio level, no matter what the recording level.