Ireland and Spain must fear that UEFA will move the European Championship matches in Dublin and Bilbao.
The countries could not guarantee on Wednesday that they can allow spectators to the games.
UEFA has postponed the deadline for the decision on the host cities to a week later.
On April 20, UEFA was scheduled to take a decision on the proposed host cities, but that deadline was postponed to April 28 on Wednesday.
On Wednesday, the twelve host cities had to submit a plan for admitting the public to the European Championship matches.
It seems almost impossible in advance that games will be played in empty stadiums.
"We informed UEFA today that given the pandemic, we are not in a position to provide assurance on a minimum number of spectators," Irish association FAI said in a statement.
"We will continue to discuss the situation with UEFA."
Bilbao had previously submitted a plan to UEFA on Wednesday to fill the San Mamés stadium with a 25 percent audience during the European Championship, if contamination figures were to fall.
The Basque town was called back later in the day.
The Spanish football association RFEF reports on Wednesday evening in a statement that the seven conditions set by the local government are "impossible to achieve".
"Yesterday we received the demands of the Basque government and we immediately labeled them unacceptable," the union writes.
Ireland's national team was unable to qualify for the European Championship.
Duels in group E between Slovenia, Poland and Sweden are planned in Dublin and a game in the eighth finals.
The other matches in group E were planned in Bilbao, in which Spain will compete against Sweden, Poland and Slovakia in turn.
There would also be an eighth final in the Spanish city.
See also: European Championship duels in Amsterdam in principle with at least 12,000 fans
Many other host cities want to admit 25 to 50 percent public
Earlier on Wednesday, the KNVB announced that it plans to fill the Johan Cruijff ArenA for 25 percent during the European Championship.
This means that each of the four duels can be attended by twelve thousand spectators.
Italy, Denmark, Scotland, Russia and Romania also submitted a plan to UEFA with between 25 and 50 percent filled stadiums.
The plans of Germany, Hungary and Azerbaijan for admitting the public are currently not yet known.
This also applies to England, where, among other things, the semi-finals and the final battle must be played.
It is expected that the Wembley stadium in London can be half filled.
The opening match of the European Championship between Italy and Turkey is scheduled for June 11 in Rome.
The final must be played in London exactly one month later.