Input value is invalid

Usually that error means there is something dependent in your configuration on the option that you are changing. (it can appear in many situations and is one of the least helpful FGT error messages IMHO) Going from NAT to transparent is a pretty fundamental change. Is this box in production? If not, I would recommend wiping to a factory config, changing to transparent before doing any other config and then continuing

I agree that doing a ' exec factoryreset' will be the most efficient way to do this. But be aware that even after a factoryreset, you might need to remove policies or such. Some stuff is configured by default, namely a DHCP server on ' internal' , a policy ' internal' to ' wan' and some address objects (as far as I remember).

I had done a reset but did not remove any of the default polices. I will give that a try and let you know. Thanks again for your help.

besides a factoryrest I had to clean up a bunch of the default configs like ede-pfau sugguested. My device is now in Transparent mode. Thanks for all your help.

Had this same issue upon upgrading a FGT to 5.2.1. When I tried to enable transparent mode via the CLI it complained that it couldn' t because WAN1 had auto-ipsec enabled:

allowaccess of interface wan1 can' t enable auto-ipsec in transparent mode.  node_check_object fail! for opmode transparent  Attribute ' opmode'  value ' transparent'  checkingfail -651  Command fail. Return code -651

I disabled the ' Auto IPSEC request' administrative access option for both WAN1 and WAN2 and was then able to change to transparent mode. So for 5.2.1 I was able to leave the default policies, DHCP server and addresses, but needed to disable the auto ipsec request on the wan interfaces.