Registers can be used by only one licensed user.  If you try to log into a register but are shown a message that its 'in use' - then there must be another user logged into the register.

To unlock the register you will need to go to Back Office->Settings-> Under Store Settings select Registers and from the opened tab click on the unlock button:

To avoid this issue, please be sure to press the 'Release Register' button from the front register Action menu before logging out.