I would recommend to create a nostr key pair (nsec... + npub...).

Every client should easily be able to do that.

The just enter the nsec... into a client, for example primal (mobile or web).

If this doesn't work well, try another client and put the nsec into it.

They can do this with every client they like.

Some recommendations:

amethyst (android)

nostrudel (web)

nostr.rocks (web)

damus (apple) (I have not tried this personally)