Translation:Yo tengo un gato.
I have a question. I just assumed that 'tiene' is 'has' and 'have'. But does 'tengo' mean 'have' and 'tiene' mean 'has'?
Kind of. Tener is 'to have'. It's an irregular verb, which means it doesn't conjugate in a regular way like eg. comer, to eat would: yo como, él come, tú comes, ellos comen etc.
Relook at your tener conjugation for present tense:
yo tengo - I have
tú tienes - you have
él tiene - he has
ella tiene - she has
ellos tienen - they have
Nosotros tenemos - we have
Why isn't acceptable tengo without yo? In the old version the game accepted. In the new version why not?
They are looking for more proper writing. Yo tengo is the proper way of saying it.
Overuse of subject pronouns is a common mistake made by non-native speakers. Using the "yo" isn't more proper, but it does add an emphasis to the word "I" in the sentence. It has the same effect as saying "I have a cat".
It's more likely an oversight in the programming for DL. You should report it.
FULL DISCLOSURE: Native English speaker - US, Southern Appalachian dialect. Other uses of English may vary. Advice about Spanish should be taken with a grain of salt.