Depends on what version of Lightning you installed. If you install the matching Lightning version for the Thunderbird version it will work.

https://addons.mozilla.org/en-US/thunderbird/addon/lightning/versions/

check this: are all your calendars set to read-only in the calendar properties dialog?

When I used the "get add-ons" function within TB V.29 Beta, the "recommended" Lightning add-on version was several versions old. Mozilla should take whatever steps are necessary to ensure that the links provided within that section of the software only links to the most current, and fully-compatible version of the add-on.

That the provided link directed me to download an archaic version that was incompatible is nonsense, and leads to a lack of satisfaction.

Thunderbird Releases are still based on Tb 24 as the current is 24.5.0 as to why version 2.6.4 is being served on https://addons.mozilla.org/en-US/thunderbird/addon/lightning/

For the beta test builds of Thunderbird which are built for QA purposes, you need the appropriate lightning version as it is 3.1b1 for Tb 29.0.

I'm using the latest release v.24.5.0

When I open 'Add-ons Manager' and use the search for 'Lightning', the list returned shows 'Lightning v.2.6.4.

What version does it show in your TB 29 beta ?

If it was the version as described by James then that would explain why.