Is it theoretically exist a computer that never affected by computer virus?
Do you mean "that cannot be affected" or "that was never affected"?
Then, under what conditions? Do you mean malware in general or a more
restrictive notion of virus? And then, what is virus or malware? Is
code for automatic update of your OS considered malware or not? Maybe
you do not want to update your OS. Maybe someone has taken control of
the updating service to introduce undesirable features in your system.

The main problem with your question is probably the word
"theoretically". What is a theoretical computer?  If it is a Turing
Machine, the answer is clearly yes: they do what their finite control
says they should do. Period.

Is there a fully precise theoretical model of a real computer. That is
doubtful, if only because it is a complex physical objet and there is
a limit to our ability to fully model such an object, since
our knowledge of physics is not complete ... though I doubt that is
the main issue.

Assuming we could completely model the physical computer with a formal
system, do we fare better? Now we have (or shall have) all these great tools for
proving theorems about computers and programs. But this has limitations too. Gödel
says that not all true facts are provable, especially in a theory that
can model that much computation. But even without falling in this
black hole, do we fare better?  No because before proving anything
about the computer we need to model precisely what a computer is, and
also what it means to be affected by a virus. Even if we have a
perfect and total proof system (which we do not), how do we prove that
our theoretical specification of the problem is accurate with respect
to the physical problem with real computers. Bugs are not the
exclusive privilege of programs and proofs, you can also have them in
specifications, and some may be hard to eradicate. If your child
specifies he wants a train as a gift when it is actually a car that he
wants, what are the formal tools that will identify the specification
error - short of implementing and observing that things do not behave
as expected.

Coming back to a more practical view. Some machines are harware
protected with encryption keys that allow only trusted software.
First, many people consider this as malware by itself, as it is often
used against the owner of the computer. Then encryption can be
broken by mathematical or social means. And the protection may possibly also be circumvented by other means.

A computer that stays off-line and does not access foreign data is
unlikely to be affected. Then, there are different ways of accessing
foreign data that may be more or less risky. Using foreign code is
likely to be more risky. So the question cannot be independent of
defining  the conditions in which the computer is used.

However, even off-line, there is always the risk that the computer was
born affected by malware (to avoid the restrictive concept of virus).

You should read Ken Thomson's 1984 Turing lecture: Reflections on
Trusting Trust.

Either you trust others to some extent, or you rebuild most of the
technology from scratch by yourself, with no interference from others. May be not starting at stone age, but far enough ... assuming you do not have bugs in your own code (but if not intentionally malicious, they are less likely to be as much a problem).