Error in sub-node ‘Simple Memory‘

"I’m having a problem when trying to use a simple memory name in my flow. The ‘Key’ field doesn’t work when I insert an ‘expression’ (variable), only when I insert a fixed value.

When I try to run the flow with the ‘Key’ having a fixed value, it works. When I insert any variable value, it gives the following error:

Error in sub-node ‘Simple Memory’"

(all nodes arte connected to memory)