Runtime Error MIT App

Hi everyone! May I ask, what is the meaning of this and how can I fix it? Thank you!

This means that one of the text boxes is empty. The addition block only accepts numbers, not blank fields.

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Thank you for your answer. May I ask you again, what built-in function will I use if I want to have conditions like if the input is 20-30 then the result or the number that is going to appear is 10.

There is an if condition in the control block drawer on the left. There is a <> = comparison block in the math blocks. There are OR and AND blocks in the logic blocks.
After you combine all these blocks, you will get what you want.

I think you can't add 100 + "100" , at least in java.
(Both are different objects)
Either you will get an error or "100100" .