If you have not received merits from a brawl, there may be a few possible reasons for this, outlined below:
Is the brawl over yet?
If not, then please wait until the brawl cycle is over. You will receive your merits following the end of the current brawl.
Is your brawl stuck?
If so, then please refer to this guide: Our brawl is stuck! What do we do?
Did you submit your brawls, or flee from your brawls?
To get full merits, you need to submit and not flee from at least 75% of your brawls. Otherwise, the amount of merits you receive is proportional to the number of brawl battles you submitted and did not flee from.
You will not receive any merits when you do not submit or flee from all your brawl battles. If you submit and lose all your brawl battles, you will receive full merits because losing is not fleeing.
If you are receiving fewer merits than your guildmates in a guild you have recently joined, please refer to this guide: Why Did I Receive Fewer Merits Than My Guildmates?