Is it permissible to pray before the beginning of its time due to an excuse?

It is not permissible to perform the prayer before its prescribed time, and it is not valid; for Allah Almighty says: "Indeed, prayer has been decreed upon the believers a decree of specified times." (An-Nisa 4:103). And whoever intentionally does so, their prayer is invalid and they incur sin. If it was unintentional and they thought the time had begun, then they do not incur sin, and their prayer is considered a voluntary one (nafilah), and they must repeat it.