Desktop or phone?
I've never been able to do it on my phone.
On desktop I got that error for years and finally contacted MCPS tech support who shared that the link had been incorrect (though I honestly think it was their link?) so once I got the right link it worked.
I also learned today that you can opt in to having your child's scores shared in the "this assignment has been graded" emails that are generated from Canvas. That takes away 90% of my reason to even try to log in to Canvas.
Here's how.
Go to classroom.mcpsmd.org
Click Parent Log-In
Log in to ParentVue
Click on the My MCPS Classroom button on the dashboard
Click on "go to MCPS Classroom in browser"
I was already logged in, so you might have to sign in to My MCPS Classroom/Canvas at this point.
Click on account.
Click on notifications. That takes you to notification settings, and if you scroll down you can toggle underneath "Grades" to allow the grades to be shared in the email.