Hi @Toh_Yu_Kai,
By default, the variable accepts a text string datatype - both letters, numbers, and punctuation - that’s why you see the double-quotes " 100 ". However, if you want to use the change variable by number block, you need to set your initial variable to a number datatype. This is available in Data > Values and looks like this:
Drag the number value CoBlock into the value space in your variable and change the value so it looks like this:
Now your change variable by number blocks should work.
Hope that helps! Let me know if you have any further questions.
Geoff @ TechLeap
PS: If this solves your problem, please mark as Solved.