It shows a active license on the customer portal but not activated on the sem application

Check the spelling of the machine name.  If you continue to have issues you may need to open a support ticket and have your licensing cleared.  

Did that already.

The license was cleared. I activated the license again but still face the same issue.

i did check the machine name and the machine id several times. It was found to be the same as entered on the customer portal.

The unique key might be the issue.  Look carefully at the number 1 verse the lowercase L.  Also the zero and capital O are confusing as well.  This is an older key format.  Is this for ETS or KIWI?

I was carefull about the unique key and did emphasize on the confusing numbers and letters but still i try it once more. But i doubt it will resolve the issue.

This activation is for SEM.

Ok, validate everything one more time, open a ticket with support.  I have not used SEM in about 2 years.  

Let us know how this goes... I also am still using SEM on multiple networks and haven't seen this yet.  I also can only register offline.

I had this issue awhile back and had to register offline as well

Did you solve this problem? I'm currently having the same issue. My license was reset  and the Unique ID and hostname were triple checked. 

How did you resolve this?

Raise a support case with SolarWinds, get a new license key/file from SolarWinds support. They can deregister your current licenses and get a new license since you already have paid for it. I dont use SEM, but I had this issue on other SolarWinds modules in past and this is how I got my issue fixed, by asking them to generate new licenses for me.