We all make a little mistake sometimes....
That is probably why your CSR was rejected.

Error: Common Name mismatch
This mean the Common Name (CN) on the CSR and the domain name entered into the order do not match

Fix
Check the domain for which you ordered in your Sectigo portal. Is there a difference between the two?
If the mistake lies in the order, simply submit a ticket here: https://sectigo.com/support-ticket
Choose Certificate Technical Support from the drop down menu.
Attach a new CSR with the correct domain name, be sure to include your order number, and let us know to change the domain name.

If the mistake lies in the CSR, simply generate a new CSR with the correct Common Name (CN).

Error: Unsupported Key Size
All CSR's must have a minimum of a 2048 key size.

Fix:
Simply generate a new CSR with the correct key size

Error: No Country Code
All CSR's must have the country code listed [i.e. US (United States), UK (United Kingdom), etc...]

Fix:
Simply generate a new CSR with the country code listed