Butina is detained in the United States on charges of conspiring to act as a foreign agent.
"I made two attempts to visit her this week, but was unsuccessful. I don’t know why," Potapov said on Thursday.
The administration of the detention facility in Alexandra, Virginia, where Butina has been placed, said no visitors are permitted on Thursdays because state government inspectors visit the facility then.
Potapov said the person in charge at the detention facility told him to come again on Friday.
"Orthodox Christians have Holy Friday that day and there is no way that I can visit Maria because we have wall-to-wall services in the morning, afternoon and in the evening," Potapov said.
However, Potapov noted that was able to speak with Butina over the telephone and said she was very disappointed that he was unable to visit her.
"Maria was very much counting on praying, receiving Holy Communion," Potapov said.
Speaking of Butina's emotional condition, Potapov emphasized that she is "very anxious" ahead of the sentencing hearing on Friday.
"She wants to go home to live with her parents and grandmother," Potapov said.
In November, the Russian Embassy in the United States said US authorities kept Butina in a single cell under solitary confinement for 67 days, exceeding the limit recommended by the Nelson Mandela Rules for the treatment of prisoners.
Russian President Vladimir Putin has said Butina’s indictment was unjustified because she was not charged with any mission on behalf of the Russian government.