I have this local map that'd I'm trying to add a grid with decimal points. The internal grid won't show, also when I add the zebra frame the white parts only appear in the corners (as tiny squares). I can't figure out what I'm doing wrong.
In your example you are using 100 degrees in X and Y. I can see that from the projection you are using (EPSG 4326).
Guessing you want to present a grid in km, you need i) to find out a CRS suitable for it like an UTM projection with meters, then use map units with the interval you want.