Can I add the tuition for my online master degree to my taxes?

Yes, if the school is an eligible institution. 

 To be eligible for the tuition credits, the course must be taken at "an eligible institution". The school should be able to tell you if it is an eligible educational institution. In general, an eligible educational institution is an accredited college, university, vocational school, or other postsecondary educational institution, including accredited, public, nonprofit, and proprietary (privately-owned, profit-making) postsecondary institutions. Additionally, in order to be an eligible educational institution, the school must be eligible to participate in a student aid program administered by the Department of Education. If they issue a 1098-T they are probably an eligible institution.

Enter your school at the link below, to see if it's on the dept. of education list.

https://ope.ed.gov/dapip/#/home

Possibly.  If you are taking an online class at a school that also has a brick and mortar site and is eligible for federal student aide, then yes, you can enter your tuition for the Lifetime Learning Credit.