"They became very nervous."
Translation:Loro sono diventate molto nervose.
Because your previous sentence used the plural female form 'diventate' with plural male form 'nervosi'.You should match 'male-male' and 'female-female' together before you complete a sentence.
You should use 'diventatI' with 'nervosI' and 'diventatE' with 'nervosE'.Then it will be accepted.
Glad to help :-)