I faced your issue in WSO2AM 4.5.0 and I don't know the root cause to be honest, but it seems it is related to OpenAPI (swagger) versions. one workaround is to get the "swagger.json" file and change the version to "3.1.0" (for some reasons it doesn't work with versions like "3.0.4" ) and upload it manually.

Another issue that I encountered is when the swagger file is big, the engine loads it but during revision it fails. As I deployed the same file in my production (which has more resource and is connected to an oracle DB) it was fine.