Is it okay to be married but file single?

No, you cannot file single if you are married. Married taxpayers can only file married filing jointly or married filing separately. If you live in separate homes and children live with one or both of you in the separate homes, you may be able to file head of household.

See the following from IRS Publication 501:

Married persons. If you are considered married, you and your spouse can file a joint return or separate returns.

Considered married. You are considered married for the whole year if, on the last day of your tax year, you and your spouse meet any one of the following tests.

1. You are married and living together.

2. You are living together in a common law marriage recognized in the state where you now live or in the state where the common law marriage began.

3. You are married and living apart but not legally separated under a decree of divorce or separate maintenance.

4. You are separated under an interlocutory (not final) decree of divorce.

You can claim head of household if the following applies:

You may be able to file as head of household if you meet all the following requirements.

1. You are unmarried or considered unmarried on the last day of the year. See Marital Status , earlier, and Considered Unmarried , later.

2. You paid more than half the cost of keeping up a home for the year.

3. A qualifying person lived with you in the home for more than half the year (except for temporary absences, such as school). However, if the qualifying person is your dependent parent, he or she doesn't have to live with you. See Special rule for parent , later, under Qualifying Person.

To qualify for head of household status, you must be either unmarried or considered unmarried on the last day of the year. You are considered unmarried on the last day of the tax year if you meet all the following tests.

1. You file a separate return. A separate return includes a return claiming married filing separately, single, or head of household filing status.

2. You paid more than half the cost of keeping up your home for the tax year.

3. Your spouse didn't live in your home during the last 6 months of the tax year. Your spouse is considered to live in your home even if he or she is temporarily absent due to special circumstances. See Temporary absences , later.

4. Your home was the main home of your child, stepchild, or foster child for more than half the year. (See Home of qualifying person , later, for rules applying to a child's birth, death, or temporary absence during the year.)

5. You must be able to claim an exemption for the child. However, you meet this test if you can't claim the exemption only because the noncustodial parent can claim the child using the rules described later in Children of divorced or separated parents (or parents who live apart) under Qualifying Child or in Support Test for Children of Divorced or Separated Parents (or Parents Who Live Apart) under Qualifying Relative. The general rules for claiming an exemption for a dependent are explained later under Exemptions for Dependents .

...Also, been separated for the last 6 years, trying to get divorced.  She will not divorce me, but my lawyer should have me divorced very soon.  She hasn't lived in the same state as me & our children for the last 3 years. 

I was married till June 2019 but was separated from my spouse since 2018. Therefore I filled for taxes last year as single. After that, I got divorced in June 2019 , now I'm unable to change my status on the website. It says error 06. Is there anyway to change my status on the CRA account? thanks