This is the 383rd most frequent Chinese word.
无法
"Unable to" or "cannot."
Here, '无法' expresses incapacity caused by a specific reason, with the action '参加会议' (attend the meeting) being hindered by '生病' (illness).
他由于生病而无法参加会议。
He is unable to attend the meeting due to illness.
The word '无法' indicates an inability or impossibility to do something, showing that the action '解决' (solve) cannot be completed under the circumstances.
这个问题太复杂了，我无法解决。
This problem is too complex, I cannot solve it.
In this sentence, '无法' is used to show impossibility dependent on specific conditions, where '进入这个国家' (enter this country) is made impossible without '签证' (a visa).