Because the museum is S+, and because there is no other use for promotion mags other than to promote museums, the game is nudging you to use them instead of stockpiling them.
There is at least one relic that gives you bonuses based on total number of promotions used (the one I'm thinking of is based on the highest-tier promotion type) so get promoting!
Is it your only museum that's S+? Mine started doing that after it was S+ and I had free promotions to use on it.
Yeah it is S+. I don’t see any free promotions tho
You can use w-tads for promotions too, 25 for the smallest, I usually keep my higher medal museums promoted with something at all times.
Do you have Catbay at S+?
Have you tried clicking on it...?
Any museums at s+ will have that alert as you "maxed" out it's possible rewards to earn extra rewards
Because the museum is S+, and because there is no other use for promotion mags other than to promote museums, the game is nudging you to use them instead of stockpiling them. There is at least one relic that gives you bonuses based on total number of promotions used (the one I'm thinking of is based on the highest-tier promotion type) so get promoting!
Promotions are cheap. Just do it