I had already faced this problem, the solution consists mainly due to these problems,
1. The license file should be in the directory folder, the place where modelsim components are installed.
2. Never change the file encoding of license, in other words the format of license.
3. The directory main file shouldn't contain spaces like that of "Model Sim", it should be "ModelSim", likewise.
4. Strictly follow these videos, 90% it would be successful, for the installation purpose.
a)
https://www.youtube.com/watch?v=zdRt0_Oe9sA