Can cost of equity be less than the cost of debt?

It should not be. Shareholders are paid after lenders and have no promised return, so they carry more risk and should require more. A cost of equity below the cost of debt usually means an input is off, such as a beta that is too low or a bond yield that reflects distress.

A stock with a beta of 0.2 gets a CAPM cost of equity of 6% with a 5% risk-free rate and a 5% premium. If the same company's bonds yield 9%, equity looks cheaper than debt, which makes no sense. The beta probably understates the real risk.

Compare against the pre-tax cost of debt. The tax deduction on interest makes debt cheaper for the company, but it does not change what lenders require. The cost of equity vs cost of debt guide covers this, and the levered vs unlevered beta guide helps rebuild beta from peers.