Hello,
I would like to inform you that LACP is upport starting from version 15.1X49-D50.3 in switchin mode on SRX 300 and when I checked your configruation it is in switching mode and hence it should be supported.
The error that you are getting on commit is due to a configraution issue on interface ae1 unit 0. Please use the below commands and then try to commit if it allows you to commit the configraution.
# delete interfaces ae1 unit 0 family ethernet-switching port-mode trunk
# set interfaces ae1 unit 0 family ethernet-switching interface-mode trunk
Hope this helps.
Thanks,
Pulkit Bhandari
Please mark my response as Solution Accepted if it Helps, Kudos are Appreciated too.