I found this article for the transfer window for September, 2020 0 not sure if it applies for the March 2021 window:
Believe I read where they've changed it this year, teams can withhold their players.
"The situation of the pandemic is rapidly evolving across the world and many national governments have again implemented travel and immigration restrictions due to a renewed increase in COVID-19 infections. Some of these measures directly impact international competition, such as mandatory periods of quarantine or self-isolation and travel restrictions.
As a result, in order to ensure that the well-being and health of all individuals involved in international competition is respected, the Bureau of the FIFA Council has, after consultation with UEFA, decided that the general rules which normally oblige clubs to release players for national team matches should not apply in the following instances
:there is a mandatory period of quarantine or self-isolation of at least five days upon arrival at:
the location of the club which has an obligation to release the player to an association team; or
the location where a representative team match is scheduled to take place;
there is a travel restriction to or from either location (a. or b. above); and
no specific exemption from the relevant authorities relating to the above decisions has been granted to players of a representative team;
"If the same exceptions are applying in the upcoming window, is Portugal on such a list for UK? What about Brazil, Spain, Turkey, Senegal or Egypt?