The next letter you can solve for is the letter S in the top row. S can be the numbers 8 or 9.
If it were an 8, and you added it to the number 1 underneath it, that means you would have to be carrying a one from the previous column, where the letter E and number 0 appear. The only number that E could be in such a situation is 9. When you add that to 0, the only possible results for the letter N would be the numbers 9 or 0. Since E and N both can’t be 9, and it can’t be the number 0 since that has already been used by the letter O.
That means S must be the number 9.
At this point, the solution would look like this:
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