Opponents of determinism say that predictions are often not possible.

That is probably true.

But it is also true that identical substances always show identical results under identical circumstances. Because everything consists of substances that work according to the law.

 
Three rule-determinism:

1. Determined (i.e. predetermined) is everything that takes place according to laws (because the laws definitely determine the flow of substances).
2. “Law” means that identical substances always produce identical results under identical circumstances. (Laws are properties of substances that - viewed in isolation - are immutable unless something is added to or removed from the substances).
3. "Substances" are neurons, brains, cells, living things, elementary particles of the standard model of physics, atoms, molecules, neutrinos, crystals, liquids, gases, stars, star systems, galaxies etc. - everything in the universe.

In addition, they are also considered to be virtual substances in the states of empty spaces  (the vacuum in space).

 Wikipedia
https://de.wikipedia.org/wiki/Determinismus
 
“Deterministic systems are completely determined by the initial conditions, but are only partially determined; Determination is defined as the degree of “predeterminedness” of such systems, closely following a predictability. Both practical and fundamental limits with regard to the accuracy of the measurements or the calculation steps limit a prediction. "
 
My comment: Those who only allow exact predictions are disregarding rule

No. 1
 
People who want to prove indeterminism keep making the same arguments: From the starting point A to a destination X, if both are temporally or spatially apart, usually no reliable prediction can be made. Either, you have not included all the components in question in A. And, it is impossible to calculate all the interactions to which the elements from A to X are exposed.
 
And because one therefore cannot make a definitive statement about what X looks like after a certain time, it is claimed that not everything is determined. This concerns causality, which says: Everything is determined by predecessors.
 
Nobody would doubt that identical substances always produce identical results under identical circumstances. Or to put it more loosely: the same substances under the same conditions always produce the same result.
 
It is therefore not relevant to predict results, but to refute the sentence just quoted in order to refute the indeterminism. As long as no one succeeds in doing this, determinism is a statement that cannot be falsified.
 
 
Objective chance is also used against determinism:
 
Wikipedia
https://de.wikipedia.org/wiki/Indeterminismus
 
“According to modern probability theory, the concept of objective randomness does not necessarily require the assumption of a“ metaphysical indeterminism ”, but“ can be explained by what is called 'deterministic instability' in physics ”. Singularities or unstable points in the calculation models, also within deterministic, classical mechanics, have the effect that arbitrarily small differences in the initial state lead to maximum deviations in the results after a sufficiently long time. The result is determined by "immeasurably small fluctuations and is therefore impossible to predict". Together with the fundamental limits of exact measurability, this implies "the existence of objectively indeterminate processes even in the size range of macrophysics."
 
My comment: Here, too, it is not taken into account that "any small differences in the initial state" also proceed according to laws. This is especially true for quantum physics!
 
In all cases, only the criteria given by humans are classified as valid: measurability and predictability - and not that everything runs according to laws.
 
This also applies to probabilistic statements:
 
Wikipedia
https://de.wikipedia.org/wiki/Probabilistische_Aussage
 
“A probabilistic statement (also: probability statement) says about a fact (such as the occurrence of an event) that it exists with a certain probability. With the exception of logical conclusions or conclusions based on strictly deterministic laws, in many cases there is insufficient justification for claiming certainty for certain facts. Often, however, absolute or relative, exact or approximate probability ratings can be given; so only probability statements are possible. Such statements generally do not allow the determination of individual cases; rather, they are statistical statements about regularly occurring events. Otherwise, however, different philosophical interpretations of probabilities are being debated. "
 
My comment: The second deterministic rule is: “Law” states that identical substances always produce identical results under identical circumstances.
 
To refute this rule, opponents of determinism need only create two absolutely identical circumstances in which there are two absolutely identical substances that, after trying, do not achieve an absolutely identical result.
 
This is hardly possible in our macro world.
 
Therefore, they switch to the micro-world (you should, however, consider properties such as entanglements with regard to quantum mechanics, which make it difficult to establish identical circumstances.)