We have migrated our one environment from Appian 7.2 to 7.6 to 7.7 by using same

This is because your DT_MODEL table wasn't in sync with your engines, you need to re-import your CDTs from a different environment

Does it need to be done for all 297 CDTs? and Do you know any alternative solution for those CDTs are not present in other environment.

For those CDTs that are not present in the DT_MODEL table. The fact that you see them in the Data Management tab means they are registered in your DESIGN engine but if you cannot download the XSD it is because there's no corresponding row in the DT_MODEL table for those CDTs

Thanks for your information , Do you know what could be the reason and how we can prevent this in future

This can be prevented by avoiding any manual updates to the DT_MODEL table, make sure the DT_MODEL is never wiped out, that this environment never points to a different primary DS, that you don't revert to previous *.kdb without reverting the primary DS too, basically avoid anything that can put this in an unsync'ed state

Thanks Ed, Reimporting is done and imported fine Now we are getting another problem while creating a new xsd, giving error below ,
2014-11-10 19:23:14,133 [ajp-/0.0.0.0:8009-9] ERROR com.appiancorp.type.admin.ImportDataTypeAction - An import exception occurred while importing the types.
com.appiancorp.suiteapi.type.config.xsd.exceptions.InvalidXsdException: The XSD could not be imported because it references a missing type: {xs}string. No types have been imported. (APNX-1-4049-001)
          at com.appiancorp.type.config.xsd.XsdValidator.validateComponentResolved(XsdValidator.java:249)
          at com.appiancorp.type.config.xsd.XsdConverter.getDatatype(XsdConverter.java:206)
          at com.appiancorp.type.config.xsd.XsdConverter.getDatatype(XsdConverter.java:152)

1. Can you describe the steps you are following to cause the error?
2. Can you import XSDs using the Import Data Type button?

I have just tested with new flow like , created a CDT using Create Data Type and then mapped that CDT into new Datastore and published ,and created Query Rule. but when we try to access the query rule getting error like (defined test rule not found) . After logout and login it works . Any suggestion

That is a cache issue that happens if you go to download the XSD of a CDT and then hit the Canel button next to the Download XSD button.

This can easily be fixed by hitting F5 to refresh the page; not an issue with your environment.

Thanks for your comment