RE: Error while importing solution from CRM 365 online to CRM 365 on-premise
Good Afternoon,
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As for selecting the solution version: this question should pop up during export (both for managed or unmanaged), after choosing the solution type. Please note, that you can not choose target version, while exporting the default solution. It also will not pop for instances on version 9 - at time of this answer.
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As for solving your issue: for being able to provide a proper answer, I would like to know also the version of your D365 instance, and whether it is an unmanaged solution or the default one, which you try to export.
If under D365 instance you refer to v9, then it never prompts the question to choose target version (yet - until a later minor release), but in this case your linked document is incorrect, see the right one:
https://docs.microsoft.com/en-us/dynamics365/customer-engagement/developer/export-solution-specific-version
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... And for a not proper answer/hack... in the exported zip, in the solution.XML you can see, which version the solution is targeting. "Hack" could be playing around with this tag, but I strongly advise against it, as it is unsupported to mess with the solution.XML!
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Little bit detail: newer versions of CRM usually itroduce fresh capabilities, which are not part of previous versions. When you identify the target version, CRM removes solution components from the exported ZIP, which depend on newer capabilities than the targeted version can offer. If you start to play around with the solution.XML, but your ZIP still contains components, which should not be there ("introducedversion" tag in customization.XML), you may end up with a corrupted solution, or even after the import (if it will be at all succesful), a probably corrupted instance.
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Best regards