You have to go to the pokemon menu and go to the pokemon with strength and select to use it. If you are doing it i guess your problem is the path you took.
>On 1F, the inscriptions talk about a brave king who fought hatred and pursued peace. In the middle of the lines, it states about the King turning on corners. These corners correspond with the intersections while entering the ruins. Every time the player comes to a choice in the path, either going straight or turning, can be considered a corner. Following this pattern, turning on the 2nd, 3rd, 5th and 7th intersects, and continuing straight on the others, will lead the player very close to the center of 1F, regardless of what entrance is taken.
This key of 2nd, 3rd, 5th and 7th corners works on all floors, (except obviously on the last floor) and will lead the player to the Relic Crown.
The inscriptions on the 2nd and 3rd floors tell the player to "shine light" and "act strong" if agreed, respectively. Both at the end of these floors, the player must use Flash and Strength, respectively.-Bulbpedia
The path you take has to be very specific you can look on youtube of videos that give you the right path just follow what they do.