您需要使用（经度，纬度）序列。因此相关代码行必须是：-

mppng.scatter(-116.70722, 31.79872, s=120, color="red")

s=120，颜色=“红色”为选项。

我还要补充一点：

mppng.gridlines(crs=ccrs.PlateCarree(), draw_labels=True)
mppng.set_title("a_posteriori_title")

才得到这幅图。
完整的代码基于OP的问题，读者可能会尝试运行：

import matplotlib.pyplot as plt
import cartopy.feature as cfeature
import cartopy.crs as ccrs

# This will be used several times
my_proj = ccrs.PlateCarree()
# lat_min, Lon_min, Lat_max, Lon_max
cstm=[31.69347, -116.88353, 31.96507, -116.57810]

def map_canvas(cstm, land=True, land_color='darkgreen'):
        fig, mppng = plt.subplots(subplot_kw=dict(projection=my_proj))
        mppng.scatter(31.77, -116.75, s=120, color="red", transform=my_proj)

        mppng.set_extent(
            [min(cstm[1], cstm[3]), max(cstm[1], cstm[3]), min(cstm[0], cstm[2]), max(cstm[0], cstm[2])], 
            crs=my_proj)

        if land:
            # may cause data download from the web
            mppng.add_feature(cfeature.GSHHSFeature(scale='i'), 
                              facecolor=land_color, edgecolor='black', 
                              linewidth=.5, zorder=10)
        return fig, mppng

# This generates figure and axes objects
fig, mppng = map_canvas(cstm)

# Try to add more fetures a_posteriori
# mppng.scatter(31.79872, -116.70722) # This has wrong order of coordinates
mppng.scatter(-116.70722, 31.79872, s=120, color="red")

# Add more features on the axes
mppng.gridlines(crs=ccrs.PlateCarree(), draw_labels=True)
mppng.set_title("a_posteriori_title")
plt.show()