i want to fix my mcs to 15 and 14. how can i do that
You really can't/shouldn't -- the MCS rate is dynamically determined based on the SNR/BER of the link, as well as falling back when the link is not in use.
If you're interested in the technical details, read https://wireless.wiki.kernel.org/en/developers/documentation/mac80211/ratecontrol/minstrel and/or search "Minstrel HT rate control"
See also
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Are there any drivers supporting a fixed mcs (for experimental use)?