Against what types of enemies can I riposte? And is my only option to dodge or roll versus other types?

Against what types of enemies can I riposte, to counterattack them? Does it only work against humans? And should I hit the riposte button when their health bar glows red, indicating that they are about to attack?

And versus monsters and other creatures, I should instead dodge or roll away instead? Would blocking at least be effective versus all types of enemies, or are there some that it wouldn't be?

There is a loading screen tip that suggests that you should dodge monsters and block/parry humans. However, many have found this does not tell the full story.

Much like how many enemies can parry your fast attacks but can be broken through with a heavy attack or otherwise, parrying works for many enemies' faster, lighter attacks. For example, with the Drowners and Ghouls, you should be able to parry their regular attacks. However, their heavier attacks--jump attacks, in this case--cannot be, so you'll just need to get out of the way with a dodge or roll. This applies to other enemies. An intuitive rule of thumb is that any enemy attack that has a considerable windup/charge or sense of impact probably can't be blocked or parried.

It seems obvious that you should dodge or roll against very large monsters, but you should probably also do so against enemies with heavier/larger weapons (including humans with two-handed weapons and the Wild Hunt) which will stun you on parries instead of opening a window for you to riposte, so dodging here is a good choice for counterattacks.

Regarding attacks that you can't parry: dodging is also helpful for sidestepping single heavy attacks/charges or backstepping short attacks/lunges and taking advantage of your position to get critical or followup hits. The game also encourages a more conservative usage of rolling given its effect on stamina, so rolling is best used to avoid sustained attacks (namely the Wraiths) or area-of-effect attacks (which I believe golems may perform) or just regain some space.

In general, I've found that you can parry basic attacks from many roughly man-sized enemies, including Nekkers and Wraiths. I can't give you much more detail or specific monsters, but you should have the opportunity to test this against an unfamiliar enemy early on in a fight (activating Quen first, if you wish) by attempting to block or parry a single enemy's lighter attack. If you take damage or Quen deactivates, you'll know that enemy's attack can't be parried. Experimenting will give you a better sense and knowledge of this, however you should be aware that there are enemies that also react differently to ripostes; some hags will have their tongues cut off with a riposte which makes them much easier to kill.

In short: basic attacks (like swipes) from medium to small enemies are often able to be parried, and special/strong attacks generally need to avoided by dodging/rolling regardless of whether it's a monster or human.

Lastly, the health bar glowing red indicates the enemy is beginning an attack, but depending on your situation, parrying immediately may or may not work. If you tap to parry, it will likely work against faster attacks but a slower strong attack may land well after the parry "window" ends. It may be better to use the healthbar indicator to dodge/roll or even block, and watch the enemy's weapon (or fists) to time your parries.