吃素 is a separable verb (离合词) meaning 'to be vegetarian' or 'to abstain from meat'. The verb 吃 and the object 素 can be split when inserting duration, measure words, or complements: for example, 吃了三年素 ('has been vegetarian for three years'). You cannot place a direct object after 吃素; instead, use it as a unit or split it.