Add nanocomposite material to HFSS

I wanna to add a new nanocomposite material to HFSS. I have relative permittivity and I need loss tangent value. How can Icalculate loss tangent? please help me.
for Er=2.8,loss tangent=?

The electric loss tangent is defined as

tan(δ) = ε''/ε'

So in order to calculate the loss tangent you need to know imaginary portion of the permittivity of your material. For materials with loss, typically permittivity is defined as a complex value where εr = ε' + jε''. If this is not known, you'll have to make an educated guess.

if ε was real, this means that ε'' =0?
my nanocomposite substrate have ε=2.8, tehrefor
tan(δ) =0?

If you would like to model it as lossless, than yes, your loss tangent is just zero. This is probably an approximation as every material exhibits some loss, but there are many applications where ignoring the loss doesn't affect the results you're interested in. Both the real and imaginary parts of permittivity are frequency dependent, so if your material is not often used in RF applications, its likely no one has put in the time to fully characterize the material. Its also safer to consider materials lossless at lower frequencies.

What is your nanocomposite and at what frequency are you simulating?