The puzzle is small but frustrating.
Here's what I've deduced so far:
- In the 36 row, there are two missing digits which sum to 9, these must be 1+8 so the remaining empty cells are 2,3,4
- In the 36 column, same thing but it could be 3+6 or 4+5 missing. There must be 1 and 2 somewhere but can't nail down where
- In the 18 row, the leftmost cell is 5 or 6, so the other two sum to 5 or 6, unfortunately that still leaves lots of possibilities
- In the 19 column, there are 4 cells which sum to 10 so must be 1+2+3+4
Alternatively maybe I just overlooked something really stupid.
I have verified all pen (blue) numbers are correct using the button in the app.
Source: Puzzle Page app