Once garnishing starts is there no negotiation with the CRA?

Cra takes about $1200 a month from my paycheck. Which I can’t afford - I am drowning. I want to call and work out a payment plan (which I was in one since 2016 and never missed a payment, they just wanted MORE money monthly and I didn’t do the reassessment). This is not my only debt, but right now it’s the only debt I am worried about. Everything else is still in good standing. I don’t want to ruin my credit and also ruin any chance of career development…but I keep reading that once garnishments start there is no negotiating with the CRA, is this true?