Tax lien for years 2011-2012 was filed with Maryland Circuit Court against my ex husband and me (we did joint tax return) in February 2015.
According to our records tax years 2011-2012 are part of the payment plan agreement established between my ex husband and Comptroller of Maryland in October 2013. The garnishment payments were being deducted form my ex husband paychecks twice a month. At the time when the tax lien was filed with the court in February 2015, the garnishment payments were being made on a regular basis. We have copies of his paystubs showing the deductions.
Also, by the payment plan agreement any tax refunds are applied to the tax debt. Therefore, tax refund for tax year 2014 was applied towards tax debt for years 2011-2012.

My question is why the tax lien was filed with the court if the payment plan was established already? Or was it filed incorrectly?

Thank you in advance

in general when you are unable to pay state for taxes you owe, you may be able to qualify for a tax payment plan.

The agency may have placed a tax lien against you as you failed to pay a tax debt. If you have unpaid tax debt, the agency can seize your assets to pay it off. It can seize money in your bank accounts, garnish a portion of your wages or keep your state tax return. It can also seize and sell property that you own. A state tax lien, like a Federal tax lien has NOTHING to do with a garnishment or levy on your wages. A lien is a notice to your creditors that you have an outstanding tax liability.

A lien damages your credit score and may bring a flood of letters from companies that research tax lien filings offering to help solve your tax problem, but it does NOT cause an action on your wages.

as you said, You could set up payment agreements with the Comptroller of Maryland Compliance Division; you can call them at 410-974-2432 or 1-888-674-0016 I guess you need to contact the agency for mor einfo in detail. Comptroller of Maryland - Public Services - Agency Information