Is it incorrect to say "If you could"?

There is nothing wrong with "If ... could ...".

(I looked at your linked rule, but can't comment on it as my German is not good enough.)
What I would say is that sometimes "rules" are given to learners as guidelines to prevent them making elementary mistakes until they have mastered more of a language. That is what I suspect happened in the related question I mentioned in a comment above: May you please explain this?.

Could is the past tense and subjunctive form of the verb can, which may be defined as follows:

can verb (past tense could)
1. to be able to • Can you lift that?
2. to know how to • He can play the guitar.
3. to feel able to; to feel it right to • How can you believe that?
4. used to express surprise • Can it really be that late?
5. used to express a possibility • The weather can change so quickly in the mountains.
6. to have permission to • Can I take an apple?
7. used when asking for help, etc • Can you give me the time?

Taking selected meanings from above, there is nothing wrong with

If you can/could ...

for example

If you are/were able to ...
If you know how to ...
If you feel able to ...

No, it is correct; you can read this article which has an example of

if you could

Example 1

Even if you could get a zero percent home loan for 30 years, if it eats up more than half your net pay, you probably can't afford it.

Example 2

What if you could capture the entire history of recorded human existence into one epic infographic for the ages?

References

reference for example 1

reference for example 2