Can a private sector employer give you a raise on a signed written evaluation, then tell you2 months later you won’t be getting one due to a error?

Yes, the employer can almost certainly to this. Employers set the terms and conditions of employment, including how much their staff are paid; an employer may give you a raise then take it away, or promise you a raise and never follow through on it. There are exceptions, which may not apply:

1) There is an actual employment contract or agreement between you and the employer, setting out your compensation and when (and under what circumstances) you get a raise; if so, it's terms will be honored. A written evaluation form is not generallya  contract, however, since you did not give up anything (provide "consideration" to bind the agreement); rather, it is a simply memorandization of a unilateral promise by the employer which, since it was a unilateral promise and not a contract with consideration on both sides, may be ignored or rescinded.

2)  Even if there was no actual contract, if you took some action to your detriment in reliance on the promise--e.g. you gave up another firm job opportunity because you thought you'd get a raise--and the employer knew or should have known you'd do that on the basis of the promise, then that may be enough to bind and enforce the promise.

3) If you are being discriminated against on the basis of race, religion, age over 40, sex, or disability.

Other than as the abve, however, the employer may promise a raise, then go back on its work.