There is a key translation issue with respect to fictio. In the Glossary to his translation of the Ethics, Edwin Curley notes:

Fictions are posits of the imagination and so fall under the first kind of knowledge, the sole source of error in human cognition (E2p41). However, in the TIE Spinoza asserts that there is no error involved in positing a candle “burning in an imaginary space, or where there are no bodies” (TIE[57]). Nor is there error in an architect’s conception of a building that never has, and never will, exist (TIE[31]). The Ethics offers a more complex account of the capacity of the mind to entertain fictiones (see Steinberg 2018b). In E2p17s Spinoza states that “the mind does not err from the fact that it imagines” and “the imaginations of the mind, considered in themselves, involve no error” (E2p49s3.B[ii]). The error occurs when one is deprived of an adequate grasp of the idea under consideration. Put differently, we do not first conceive the fiction of a winged horse and then, by a separate action of will, affirm or deny the existence of that horse. Rather, every idea necessarily involves an affirmation or negation. Spinoza rhetorically asks: “what is perceiving a winged horse other than affirming wings of the horse?” (E2p49s). If the fiction of Pegasus involves error, such error involves the privation of adequate knowledge about the nature of birds and horses. Gaining adequate knowledge is the only means through which an individual may come to deny the real existence of the winged horse. Falsity involves privation or lack. For Spinoza that which is false has no positive or real existence. An adequate understanding of an elephant will render the idea of it passing through the eye of a needle as impossible as conceiving of a round square (TIE[54]). Philosophers may reject as false the fictions of free will, final causes, and God’s judgment, because they have gained a more adequate understanding of God and nature, but this does not mean that they will also reject what Spinoza calls “beings of reason,” which might assist us to “more easily retain, explain and imagine the things we have understood” (CM1.1, i/233). The proper function of beings of reason (e.g., genus, “evil,” “time”) is to guide and regulate our judgment and reasoning but they must not be mistaken for real existents (see Rosenthal 2019).