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