This portuguese phrase is very colloquial!
Its formal form is: "Você deveria pagar suas dívidas".
As Paulo commented, "devia" (imperfect past) is commonly/informally used as a suggestion, and not as something that really happened.
I don't understand this translation of 'devia', which I take to be the imperfect. The given translations are 'have to' which I thought was 'deve' and 'should' which I thought was 'deveria' Have I missed something?
In spoken language we use devia as the same meaning of deveria, so it should fit this sentence..
Should have paid = "deveria ter pagado", this "devia" does not express past tense, even thought it, strictly speaking, is past tense.
Both would still be translated as 'pay off' in the way we speak English in England
Is dever in the imperfect tense ever used in a strictly imperfect way, or is it only used in the manner used in the solution and the comments?
Yes. An example of owing money:
• Bill devia muito dinheiro ao banco depois de comprar a casa.
• Bill owed a lot of money to the bank after buying his house.
"O pretérito imperfeito do indicativo se refere a um fato ocorrido no passado, mas que não foi completamente terminado. Expressando, assim, uma ideia de continuidade..." https://duvidas.dicio.com.br/devia-ou-deveria