If a person wants to do something but he is not able to do that due to some personal reason, what is the exact word for that reason?
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That reason could possibly be called an "obligation":
Limitation might work. One of its definitions is "the extent of one's capacity or a constraint that stops something." It could be a physical limitation, a mental limitation, or a scheduling limitation. An example sentence in this dictionary definition is: He knows his limitations as a writer.