I created my certificate using 'Digital certificate for VBA projects', then i
went to sign the macro, then save it but i get a dialogue box saying 'There
was a problem with the digital certificate. The VBA project could not be
signed. The signature will be discarded'. How do i fix it so i can save the
certificate?

First thing to be noticed about your problem is you created self signed certificate, i want to ask about the public key issued by the Microsoft for the authenticity of your certificate, if you got that then tell me, and if the problem is related to that then first get a private key for your signature.

digital certificate