My first character could purchase the Ash Fishing Rod.
My 2nd character could purchase Elm Fishing Rod but she could not purchase Ash Fishing Rod because of 'no room in the inventory' issue. She has enough inventory room in all tab including fashion (which should be irrelevant). If it is about useful item tab, the first character uses has more stuff in that tab than the 2nd character.
A fellow in PS4 forum also reported the same thing.
*** The game can read your mind. If you want it, you won't get it. If you don't expect to get it, you will. ***
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Edit: You might have to leave one potion slots empty for it to upgrade. Like I said it's been a while since I've dealt with this one.