"My cat is lying on the bed."
Translation:Mój kot leży na łóżku.
I'm afraid that without seeing what was the original sentence those words are so far off that I wouldn't even recognize the word you had in mind... If you didn't use an 's' instead of a 'z', then it would be possible, it would just be lack of Polish characters.
There's no form of "łóżko" that ends with 'ą' (the other a).
Here is the full declension: https://en.wiktionary.org/wiki/%C5%82%C3%B3%C5%BCko#Declension
The preposition "na", when it means "on something" (stating the location), takes the Locative case. That's why it's "na łóżku" here.
One of the best resources for learning cases that I found is this website: https://mowicpopolsku.com/polish-grammar/#cases