How can I sell my Share Certificate when i have a poor credit rating and am unable to open an account?

I received a share certificate when my employment ended with Aviva. I called my bank Halifax to sell it and posted off the certificate as requested. I was expecting to receive £750.
However they have sent me a letter today stating that due to my poor credit rating i am unable to open an account with them and the share certificate will be returned.
I appreciate that my credit rating is poor but I dont understand why this effects selling the shares on my certificare for a relatively low amount and it is also a one off. I wont be wanting to buy or sell shares in the future.
I am worried now that my certificate is worthless. I have looked on line for sites that can help but cant find anything.
Please does anyone have any advice on how i can release the cash from this share certificate? Any advice will be much appreciated. Thank you.

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