MP has rejected policy request from Client because this SMSID is marked as blocked

You may get MP has rejected policy request from Client(SMSID = GUID:xxxxxxxxxxxxxxx-xxxxxxxxx-xxx) because this SMSID is marked as blocked to some of your clients when a client is deleted manually from CM database or a client is blocked in the console.

Simply uninstalling and re-installing will not fix the issue. If this is a client and want to resolve follow below procedure;

1. Launch command prompt as administrator then run C:\Windows\ccmsetup\ccmsetup.exe /Uninstall
Wait until the SCCM agent is uninstalled by monitoring ccmsetup.exe and ccmexex.exe is also not present in task manager

2. Delete following folders:
C:\windows\ccm
C:\windows\ccmsetup
C:\windows\ccmcache

3. Delete following files:
C:\windows\smscfg.ini
C:\windows\sms*.mif (if present)

4. Delete following registry keys:
HKLM\software\Microsoft\ccm
HKLM\software\Microsoft\CCMSETUP
HKLM\software\Microsoft\SMS

5. Remove the 2 SMS certificates in the local certificate store
Run MMC.exe as administrator and launch the certificate panel to the local computer
Expand Certificates > SMS and Delete both certificates

6. Delete WMI namespace from powershell:
1. root\ccm Get-WmiObject -query "SELECT * FROM __Namespace WHERE Name='CCM'" -Namespace "root" | Remove-WmiObject

2. root\cimv2\sms Get-WmiObject -query “SELECT * FROM __Namespace WHERE Name='sms'" -Namespace "root\cimv2" | Remove-WmiObject

7. In Task Scheduler library, under "Microsoft" delete the "Configuration Manager" folder and any tasks within it

8. Restart the machine
9. Then reinstall the SCCM client

Monitor the MP Control component. The error should disappear from the console.