Hello does anybody know how to fix this? I'm trying to use the serial monitor to receive data from my accelerometer but this error comes up when I try to use the serial monitor.
Your topic has been moved; do not post in "Uncategorized". See the sticky topics in Uncategorized - Arduino Forum.
There are two sensible choices where your topic would fit 
- Portenta H7 - Arduino Forum because you're using a Portenta H7.
- IDE 2.x - Arduino Forum because you're using IDE 2.x
I'm not familiar with your board.
I suspect that the COM port has changed (e.g. after an upload); did you verify if it indeed still is COM6 (in the IDE and Windows device manager)?
There are a few topics about your error; it's not specific to your board. See if there is something in these search results: Search results for 'Port monitor error:' - Arduino Forum. You will have to do some digging.
Posting an annotated schematic showing exactly how you have wired it helps us help you. By showing all connections, power, ground and power sources you will save a lot of clarifying questions and time for all of us. Be sure to note any logic wires over 10/25 inches/cm in length. Post links to technical information on the hardware items including the motors, shield, and Arduino. This should include all component values or model numbers and details of all power supplies used (which could be USB power for example).
Posting the code following forum guidelines using code tags will also help. With this information we should be able to answer your question accurately. "How to get the best from this forum". How to get the best out of this forum