It is obligatory for a woman to cover her feet during prayer.
The majority of scholars hold the opinion that covering the feet is mandatory. Therefore, you must ensure that your feet are covered, either with socks or with a long garment that covers them completely.
As for the prayers you have already performed in the past, we ask Allah to forgive and pardon you.
But from now on, you should cover your feet while praying.
This is based on the Hadith of Umm Salamah (may Allah be pleased with her) — when she was asked, and it was narrated with a raised chain (marfūʿ):
“Should a woman pray in a long garment (dirʿ) and a head covering (khimār)?”
The Prophet ﷺ said:
“If the long garment covers the tops of her feet.”
Source: Shaykh ʿAbd al-ʿAzīz ibn Bāz (may Allah have mercy on him) — from his official website.