With following two conditions,
- Do not use A, if possible.
- Do not use B, if possible.
How can I make it as a sentence? Which one is right?
- Do not use A or B, if possible.
- Do not use A nor b, if possible.
Both sentences are correct. See here for some examples that use nor. Nor sounds more formal and does not typically appear in colloquial English.
Nor is used to continue a negative phrase. In modern usage, nor usually appears after neither. For example,
Use neither A nor B, if possible.
However, this example sacrifices the clearly imperative "Do not" at the beginning of the sentence. If I wanted clear and concise English, such as for "how to" instructions, I would probably use this:
Do not use either A or B, if possible.
The either emphasizes that both options A and B are to be avoided.