Hi!
What is the maximum supported resolution for your monitor? Please note that if your PS5 is running on 4K (and your TV support 4K) and your monitor only support Full HD, then it won’t work offcourse.
Please put the PS5 on your TV and select 1080P and then try again on your monitor.
If it’s still failing, then your monitor may not be compatible somehow… contact with Sony is advised if this is the case. Maybe a firmware update can fix this.