@varshinims08
In Portals, to have something that has multiple steps, stages or phases,
you should instead use a Web Form
To our awareness, Business Process Flows are for internal Power Apps Model Driven App use only, never for Portal use.
In case you think it should be feature exactly as you stated here with BPF with direct Portals exposure:
1) Check here https://powerusers.microsoft.com/t5/Power-Apps-Ideas/idb-p/PowerAppsIdeas and see if someone submitted something similar, and up-vote it. We checked and did not find it, but you should check it as well in case.
2) If #1 does not have any existing similar idea, submit a new idea over there, it may be considered for feature in the future in case you really need it exactly like that as Business Process Flow rather than Web Form.
