why find my iphone wont put my phone in lost mode?

Use Lost Mode in Find My iPhone on iCloud.com - https://support.apple.com/guide/icloud/use-lost-mode-mmfc0f0165/icloud

and:

Use the Find My app to locate a missing device or item - https://support.apple.com/HT210515

If those do not help then you will have to provide a lot more detail about what happens when you try and what error messages you are seeing. Right now we have nothing with which to work, sorry.