I do not know if the “%” was a typo but that character does nothing to the sprite sheet unless some sort of plug in is used. U/seventhirtysixam provided a decent explanation on where you can go to understand the rules for formatting but I will also drop this link to a formatting guide I used when I first started. The only way this would work is if the cells were the same size (the image inside the cell can be whatever size as long as it fits in the cell). In the middle and bottom pics you have sprites both 1 tile and 2 tiles in height (and I think width?) the program can’t read sprites on the same sheet with different heights and widths. In addition, you can’t put sprites with different cell heights in one sprite sheet.

(This is when the “$” is used, and this is primarily for singular sprite sheets) You CAN add in higher resolution sprites, but they become super fuzzy or downright unrecognizable, since RPG maker would set that resolution to the standard in RPG Maker engine. 1st row is down movement, 2nd row is left, 3rd row is right, and 4th row is facing up. You CAN use the symbol or & symbol to use slightly bigger sprites but that can look weird on the game world. The way you can use those is up to you, but if you look at the default sprites this shows an example of how the program reads them. Change the Size (4th text box down) for both X (first option) and Y (2nd option) to the size of a frame in the spritesheet (a default RPG Maker MV frame of animation is 192 x 192). Rpgmaker’s sprites are supposed to have 12 equal cells split into 4 rows and 3 columns. Undertale was not made in rpgmaker so the way those sprites have been formatted are not compatible with the way rpgmaker reads sprites.

Standard rpg maker mv sprite sheet size