No, pressing and holding the power button forces a restart, it doesn't put you into safe mode by itself. Try for longer, since it really should be extremely difficult for an app to disable this feature (unless the power button itself is broken?).
To get into safe mode you need to press some other buttons when rebooting. Since you've not told us what model tablet this is there is no way of telling you which buttons (the manufacturer isn't enough, since different models of different ages have different buttons available).
In the extreme you can force a reboot by just not charging the tablet and letting it shut down when the battery runs out. Then when it's recharged you can turn it on, and try to enter safe mode as it restarts.
Can you tell us more about this app? Because that's the other way we can try to learn something about the problem, but all we currently know is that "some app" is blocking the screen.
Unfortunately it's no longer possible to uninstall an app over the internet from the play store website (it used to be, about 5-7 years ago, but not now). If there's really no way of getting past this, i.e. even after powering down it just comes back up with this app, safe mode won't work, and you can't get into recovery mode, you may be able to use Google's "find my device" (formerly Android Device Manager) to factory reset it, but as that wipes everything it's a bit of a last resort.
