Hi elozyz,
First of all, do not try to test any workspaces in Dev environment, it's not official support. Deploy your custom workspace to UAT environment and working on testing in UAT is the suggest way.
Secondly, error message "Failed to publish Power BI Embedded report ASPurchaseSpendReportV2" is a very generic error message and will not help much on the issue identification.
Since the report shows V2, is this the first time you trying to deploy a custom workspace and experiencing this error OR there used to be some custom workspaces successfully deployed to UAT environment?
If this is the first time you trying to develop a custom workspace, I will suggest to check
1. the Power BI desktop version you are using for developing the pbix file. September 2020 version Power BI desktop is not supporting yet, you will need to use August 2020 version instead.
2. Use "Direct query" mode for your pbix file(Import mode is not supported).
If you are using August 2020 version of Power BI desktop version, I will recommend to raise a support request to Microsoft for further trouble-shooting.