"We need a big parking space."
Translation:Mae eisiau lle parcio mawr arnon ni.
With eisiau/angen there is no yn after the verb. It is an exceptional construction with these two nouns when used is this way:
Dw i eisiau car newydd (I want/need a new car)
Dw i angen car newydd (I need a new car)
[You will occasionally see a construction which explains this:
Dw i ag eisiau lifft - (literally 'I am with a need/want of a lift')
Mae e ag angen arian - (he is with a need for money)
and all that has happened is that the expressions are so common that the â/ag has been dropped in from common colloquial usage (and so Duo is unlikely accept the construction with ag).]
- Dw i'n moyn car newydd (I want a new car) - 'n/yn is required in the usual way because moyn/mofyn/ymofyn is a verb-noun (wanting/to want).
(typo? lle not llr) Yes, although Duo may prefer angen to eisiau in this context.
Or it may be that Duo wants you to use the alternative construction Mae angen/eisiau lle parcio mawr arnon ni - that is a little more formal, but both constructions can come up in the colloquial language.