Please first ensure that your Toshiba TV is an Android model, running at least Android 8.0
Telling us the model number will be of assistance,
If so, then are you following these instructions?
If not, please follow them, and tell us where the process fails, if it does.
In the event that this is not a supported Toshiba model, or anyway proves obdurate, I recommend a Roku Express, HD or 4K, £30-£50, which will add not just Now, but many other useful apps that the TV may not have.