What is it that we can't ever calculate ?

Your question is inherently ambiguous, but you may be referring to physical parameters, such as the mass of the electron, that cannot be calculated from the standard model of particle physics. These are called "phenomenological" parameters. That means you just have to measure them in the laboratory.. Ultimately, they are reducible to a large number of unit-less numbers (like the electron/proton mass ratio) from which all other properties (like the speed of light) may be calculated. There are quite a few at present (over 20). My favorite is the number of dimensions of space (which just happens to be 3). The ultimate goal of physics is to reduce the number to an absolute minimum with more general theories.

I'm with Dr R; I've no clue what you are talking about. We can calculate, find math solutions to, just about everything. We can even calculate (predict) the speed of light using Maxwell's equations for example. And we currently have equations that explain the origin of the universe...but no solutions as yet. We can also calculate (predict) expected results of events of very small things like quanta. So we can calculate just about anything there is to calculate.

But I have to believe you really mean to ask, "what is it that we can't ever measure?" Measure, as opposed to calculate.

We can't measure the very very tiny, the so-called sub atomic particles or quanta of nature. These are so tiny that we have no way of forming images or even shadows of these things because to do that we'd have to have measuring devices with a precision so fine that it can't be reached. We are talking about precision finer than 1E-35 m, a so-called Planck Length which is the shortest ;possible length in the universe.

And we can't directly measure the very very far; near the rim of the universe for example. Again, to do that, we'd need devices with a degree of precision, of fineness, that is impossible to get down to. And to make things even more difficult, at extreme distances, light doesn't even exist because photons had not been created back before 380,000 light years from the big bang.

mass. We just assume it's 0. But noone knows for sure.

life and how long you will be here to enjoy it