Your answer can be found beginning in AR 600-8-19. Generally once you have been reduced you must wait until you are eligible without wavier.
So for E-5 that would the reference is:
AR 600-8-19 paragraph: 2-3 c(3) The promotion to SPC requirements are 24 months TIS and 6 months TIMIG.
You must also be recommended by your chain of command. If you are fully eligible and not recommended from promotion to E-5 then you must be counseled as to why you are not going to the board. See AR 600-8-19.
The reduction board process can be found on page 142. If they did not do the procedures correctly you may have the ability to appeal.
Hope this helps!
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