To create a dlarray of size [1 40 1] with format 'UUU' you can use the following code:
X = dlarray(ones(1,40,1),'UUU');
If you are using the initialize function for dlnetworks, you can also create a networkDataLayout object as follows:
X = networkDataLayout([1 40 1],'UUU');
net = initialize(net,X);
If this doesn't resolve your issue, could you share your model please?