The option on NLA strips "Sync Length" (in the Action Clip dropdown of the N panel)
stops keys from being inserted if it is disabled.
This is due to the evaluation mode of the strip, which is set
to "Hold" internally but ONLY IF "Sync Length" is enabled.
Removing that condition allows to key in tweak mode regardless of that setting.
## History
This has been put in place by 89ee260ef2
Judging by the commit description, this was put in place to allow keyframing
in tweak mode. However, no explanation is given why this is only allowed
with "Sync Length" enabled. Potentially because there was no special
handling of tweak strips for keying evaluation, which has been put in
place later. (09709a7e64)
Pull Request: https://projects.blender.org/blender/blender/pulls/123902
0e8f36e63d