Can you put in a tampon when your period is light?

It depends how 'light' your flow is.

Tampons can't be worn with light flow because with less blood tampons will absorb more vaginal discharge so cause vaginal tearing that allows bacterial toxins to enter the bloodstream, which can lead to Toxic shock syndrome - less blood means more drying, thus more damage and higher risk.

If you use a light tampon and is it not saturated with blood after 4-6 hours then your period is not heavy enough to use tampons and you need to switch back to using pads. You can also use menstrual cups or softcups with light flow as they don't pose risk of TSS like with tampons.