I have been having this issue for few days now. Cordova won’t run in browser, error says browser is not added as a platform. However, trying to add browser as a platform, cause another error which says Unable to load platfromapi from platform. It also says that browser is not a valid platform. See screenshots:
Cordova issue 1
Cordova issue 2
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Answer
Remove the platform which you try to run on it (browser, ios, android, and etc.)
cordova platform rm browser
Add the platform again
cordova platform add browser
You can run
cordova run browser