Hi Dhaval,
From your post, I understand you wanted to import .mpp file to Project for the web but while doing so, you are getting an error. I will try my best to help you assist with this issue.

Any .mpp files from Project Desktop 2016 or later should be importable into Project for the web. Please note that Project for the web has lower limits than Project Desktop. The project you want to import must not exceed Project for the web limits and boundaries. If it does and you attempt to import anyway, you will get an error. I’ll suggest you please check this linked article to confirm the project is within the limit.
Please go to Project home and try importing .mpp file again:
- Browse to the .mpp file you want to import, and then select Open.
- In the Import custom fields dialog box, choose up to 10 custom fields that you want to import into your Project for the web project, then select Done, ensure that the columns are within the limit.
- The Import complete message will list any features from your Project desktop file that aren't supported in Project for the web and what changes were made to allow import. Review these changes, and then select OK to start working on your project in Project for the web.
Let me know if you can import mpp file now.
Kind Regards,
Neha