Is it ok to pledge for a specific sorority during undergrad and not get accepted but different Grad Chapter?

Yes, it's perfectly okay to pledge for a sorority during your undergraduate years and not get accepted, then later be accepted into a different (graduate) chapter. Sorority recruitment processes are independent between undergraduate and graduate chapters. They have different membership criteria, different pledge classes, and different member bases. Your undergraduate experience (or lack thereof) in a specific sorority doesn't preclude you from joining a graduate chapter of the same or even a different sorority later.