TIA GET Function

Trying to setup comms between S7 1500 and S7 1200.
S7 1500 writes 20 bytes to S7 1200.
It's not writing to 1200.
Error code is 1.

GET/PUT boxes are ticked on both.
DB324 optimize box unticked on both.
I dont know why the PUT function block is not green when enabled.
Help please.

Status 1 and Error 1:

Communication problems, for example
Connection description not loaded (local or remote).
Connection interrupted (for example, cable, CPU off, CP in STOP mode).
Connection to partner not yet established.

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Is connection ID 100 defined in the device configuration on both devices, and loaded ?

There is no right way of doing this, but I prefer to read tags rather than writing them when using GET/PUT and include a rolling counter. The reason being that GET/PUT will work on the initiating node even if the other is in STOP mode. With a rolling counter you can understand whether the comms are broken or the PLC stopped on the other end.