I think the formula might need to look like this:
%7BRoot%7D%3DCan't+Process%3A+Missing+Acct+%23
And so the full URL would be:
https://api.airtable.com/v0/appKJvw9LuKiqGcRt/tbl7MUhoqb9r5hk6V?filterByFormula=%7BRoot%7D%3DCan't+Process%3A+Missing+Acct+%23
Try using Airtable's API encoder to help: https://codepen.io/airtable/full/MeXqOg