Grammatically correct: I apologise but I cannot come on weekdays. I have
to go to a work meeting. I have known about it for
two months so I cannot cancel it. Therefore I would like to ask you if
I could come at the weekend.
[Schůze je countable, tak a work
meeting. Ask you je v pořádku. Request is followed
only by a noun (request a refund, request (some) information, request (some)
help), not request if.]
More idiomatic: I'm sorry but I can't come during the week as I have to go
to a meeting for work which I've known about for two months (nebo which was
scheduled/arranged two months ago) so I can't cancel it. Could I
(therefore) ask if I could come at the weekend (instead)?
Na “therefore I would like to ask” není nic gramaticky
špatného. Je ale hned poznat, že jde o doslovný překlad z českého
jazykovo-kulturního kontextu. We don't really say it like that in English in
this kind of socio-linguistic context and register.