NOTE
If you are using make.powerapps.com to import your unmanaged solution update and get this error Microsoft is aware of the issue and is working on a fix. To work around the issue open the Advanced settings and select the option "Update (not recommended)".
Error code 80048539 indicates an invalid patch is being imported. There are a few potential causes including the following:
Cause 1: If you see the error details mentioned in the Symptom 1 Details section mentioned above:
The solution patch version is lower than the version of the solution already installed. For example: A solution named SolutionA is already installed with version 1.1.0.0 and you are trying to import a patch that is version 1.0.1.0.
Cause 2: If you see the error details mentioned in the Symptom 2 Details section mentioned above:
This can occur if you already have an upgrade imported for that solution but it has not yet been applied. After importing a solution that is an upgrade of an existing solution, the end of the solution import provides a button to apply the solution upgrade. If you did not click that button during a prior import of a solution upgrade for this solution, you need to locate the solution in your solutions list and click the Apply Solution Upgrade button.