This is what it says it is in the error message.
You have an app that requires an active licence in order to function. As a well-behaved app, Team Calendars, like most, only stops functioning in a minimal way - it continues to display your existing data, it keeps the configurations you had for it, and it does not delete anything. But because you no longer have the right to use it, it has shut down edit functions.
Like most apps for Atlassian stuff, it does not have the "perpetual" licence that the core applications have, it has a standard "can't use it unless you pay for it" licence.
You will need to talk to your system owners about getting the subscription renewed, and a licence installed. As soon as a new licence is installed, you'll be able to use it all again (no need to restart or do anything special, just put the licence key in)
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