eisiau is a noun meaning 'a need, a want'. While eisiau can be used for either English word, the word angen is also commonly used for 'a need'.
In this particular sentence, though, the English phrase 'There is no need...' is more usual in the context than 'There is no want...'
I answered, "I don't want to go now." and was marked incorrect. Could someone please explain why my answer was wrong?
Does dim... - 'There is no...' (No mention of a person)
Dw i ddim... - I do not...
Dydy hi ddim... - She does not...
Dw i ddim eisiau mynd... - I do not want to go...
Depending on the context, perhaps. gadael is the usual, more specific word for 'leaving'. There is an overlap between the two words in Welsh as in English.
I ask because "There is no need to leave now." wasn't accepted as correct.
That is because the usual meaning of mynd is 'going, to go', and so we generally stick with that.
We cannot cover every meaning of all the words introduced in Duo. Many words have wider or less common meanings depending on their context, and these can be found in a dictionary or thesaurus.