Hi Aditi,
The SBML model in the supplemental material of the paper you are citing imports without error on my computer in MATLAB versions R2017a - R2021b (current). I don't see an attachment to the question, but the error message you are seeing indicates that the content of the SBML/xml file is invalid. I have seen this error with invalid SBML/xml files that omit the xml prolog:
<?xml version="1.0" encoding="UTF-8"?>
Does your xml file include this declaration? If it is included and the error persists, can you post the Matlab version you are using and attach the xml file?
Thank you,
-Florian