Error: Discrete value supplied to continuous scale in ggplot + geom_scatterpie

Question

I'm trying to do a map with ggplot and geom_scatterpie function but I'm keep receiving this error: "Error: Discrete value supplied to continuous scale"

the code is the following:

ggplot() +
  geom_tile(data = my_raster, aes(x = x, y = y, fill = values)) +
  scale_fill_gradientn(colours = c("white", "white", "white", "white", "grey", "white", "white"), na.value = "white", guide="none")  +
  geom_sf(data = europe_cropped, size = 0.1, color = "black", fill = "white", alpha = 0.4) +
  geom_scatterpie(aes(x = long, y = lat, r = 2), 
                  cols = colnames(my_table[2:5], color = "black",
                  data = my_table)  +
  ylab("") +
  xlab("") +
  coord_sf() +
  theme_few()

I've tried to run it without the geom_tile and scale_fill lines and it works but I need that raster as base of the map. What could be the problem the gives the error?

edit:

head(my_raster)

x	y	focal_max
-9.979089	64.98143	200
-9.965076	64.98143	200
-9.951063	64.98143	200
-9.937050	64.98143	200
-9.923037	64.98143	200
-9.909024	64.98143	200

head(my_table)

site	group1	group2	group3	group4	lat	long
site1	0.317	0	0.0448	0.280	47.7	5.08
site2	0.370	0	0.0359	0.319	47.8	5.07
site3	0.344	0	0.0269	0.233	47.8	5.12
site4	0.317	0	0.00896	0.342	47.8	5.06
site5	0.291	0	0.0538	0.373	47.8	5.04
site6	0.476	0	0.0179	0.490	47.7	5.12