I am so confused between the unit of nA, pnA, enA, and pps.

pps = particle per second.

pnA  = particle nanoampere.
The electrical current in nanoamperes ($10^{-9}$A) that would be measured if all beam ions were singly charged. i.e. neglecting the actual charge state. $1 \textrm{pnA} = 6.25 \times 10^{9} \textrm{ions/second}$

enA = pnA × Charge state.

In this sense, enA = nA, the actual current carried by the beam.

In General

$\textrm{pnA} = \textrm{pps} \times e \times 10^9$

$\textrm{enA} = \textrm{nA} = Z \times \textrm{pps} \times e \times 10^9$

where $e$ is electron charge.

For example, 12C with charge state of 6+ at $4 \times 10^{10}$ pps.

$= 4 \times 10^{10} \times e \times 10^{9} = 6.41 \textrm{pnA}$

or

$= 6 \times 4 \times 10^{10} \times e \times 10^{9} = 38.5 \textrm{nA} = 38.5 \textrm{enA}$

An ion beam of 5+ charge state at 1 pnA.

$= 1 / e 10^{-9} = 6.24 \textrm{pps}$

or

$= 5 \textrm{enA} = 5 \textrm{nA}$