Looking at your question and at your images - I believe you are in a preview that us none employees don't have access to.. Custom pages aren't due to be available until December https://docs.microsoft.com/en-us/power-platform-release-plan/2020wave2/power-apps/makers-add-custom-page-model-driven-app-designer
and if you are in teams I don't think project oakdale is in public preview yet
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