Hi Goran,
The error in the question occurs when the 'augmentedImageDatastore' function uses an underlying 'imageDatastore' with numeric labels. It is to be noted that the documentation of 'augmentedImageDatastore' shows the use of the below syntax containing 'imageDatastore' only for classification problems:
auimds = augmentedImageDatastore(outputSize,imds)
To use 'augmentedImageDatastore' for regression problems, you can use a different syntax that supports regression. For example, the below syntax uses an input table 'tbl':
auimds = augmentedImageDatastore(outputSize,tbl)
The table 'tbl' contains predictors and responses such that the first column of the table is paths to images, and the second column is scalar regression targets.
Please refer to the following documentation on 'augmentedImageDatastore' for more information on the above syntax:
Hope this helps you with your work.