Most TVs won't let you access an input when there is no video present.
If your TV has an AV input you can connect the cassette deck to the audio inputs associated with it and try it to see if your TV will allow you to select that input. If not then you will need to also connect video from another source to get the TV to allow you to use the input.
In most cases it isn't worth the trouble since internal TV speakers are pretty bad.
If you have a soundbar with an analog input then you can connect the cassette deck directly. If you use that input for the TV you can get an input selector so you can leave the cables connected.