I had a similar issue and found out the same as jzol explained in his answer: for SAGA GIS algorithms, the grids have to match in both extent and cell size (by consequence, they also match in resolution/dimensions).
Even if it looks like your raster grids match, it is worth a look on their extent and cell size, because even slight differences can cause this error in SAGA tools.
I fixed this issue by using the tool Warp (reproject) from gdal, on QGIS 3.16, and choosing the other input raster as the extent for the resampled raster. I did not reproject the raster, as I only used Warp for resampling (do that by choosing the output CRS as the same CRS from the input).
If the problem lies in having different cell sizes, you can adjust the cell size of one of the rasters to match the cell size of the other one. You can also do that by using Warp (reproject).
I wrote a blog post in which I discuss this error, feel free to check it out: https://www.luisalucchese.com/post/saga-gis-error-catchment-area/