This is reported by Sky News, citing a statement from Buckingham Palace.
“After being advised to rest, the queen goes about her simple duties at Windsor.
Her Majesty regretted that she would not travel to Glasgow to attend the evening reception of the COP26 Climate Summit on Monday 1 November, ”the statement said.
As specified, the queen is "disappointed that she will not be able to take part in the reception, but will send a video message to the assembled delegates."
Earlier it was reported that Elizabeth II returned to her duties in Windsor after spending the night in the hospital.
The 26th Session of the Conference of the Parties to the United Nations Framework Convention on Climate Change COP26 in Glasgow, Scotland will bring together over 120 leaders of states.