Hello Colin,
Thank you so much for your advice.
I have tried to move temp folder in mnt (I needed to force it under sudo), but it is still not working 
/home/ale/anaconda3/envs/qiime2-2021.2/lib/python3.6/site-packages/q2_feature_classifier/classifier.py:102: UserWarning: The TaxonomicClassifier artifact that results from this method was trained using scikit-learn version 0.23.1. It cannot be used with other versions of scikit-learn. (While the classifier may complete successfully, the results will be unreliable.)
warnings.warn(warning, UserWarning)
Traceback (most recent call last):
File "/home/ale/anaconda3/envs/qiime2-2021.2/lib/python3.6/site-packages/q2cli/commands.py", line 329, in call
results = action(**arguments)
File "", line 2, in fit_classifier_naive_bayes
File "/home/ale/anaconda3/envs/qiime2-2021.2/lib/python3.6/site-packages/qiime2/sdk/action.py", line 245, in bound_callable
output_types, provenance)
File "/home/ale/anaconda3/envs/qiime2-2021.2/lib/python3.6/site-packages/qiime2/sdk/action.py", line 418, in callable_executor
spec.qiime_type, output_view, spec.view_type, prov)
File "/home/ale/anaconda3/envs/qiime2-2021.2/lib/python3.6/site-packages/qiime2/sdk/result.py", line 267, in _from_view
result = transformation(view, validate_level)
File "/home/ale/anaconda3/envs/qiime2-2021.2/lib/python3.6/site-packages/qiime2/core/transform.py", line 70, in transformation
new_view = transformer(view)
File "/home/ale/anaconda3/envs/qiime2-2021.2/lib/python3.6/site-packages/q2_feature_classifier/_taxonomic_classifier.py", line 86, in _2
tar.add(fn, os.path.basename(fn))
File "/home/ale/anaconda3/envs/qiime2-2021.2/lib/python3.6/tarfile.py", line 1954, in add
self.addfile(tarinfo, f)
File "/home/ale/anaconda3/envs/qiime2-2021.2/lib/python3.6/tarfile.py", line 1982, in addfile
copyfileobj(fileobj, self.fileobj, tarinfo.size, bufsize=bufsize)
File "/home/ale/anaconda3/envs/qiime2-2021.2/lib/python3.6/tarfile.py", line 252, in copyfileobj
dst.write(buf)
OSError: [Errno 28] No space left on device
It is still using tmp as the temp folder, it doesn't let me use the space I have in mnt.
I guess the one and only solution could be changing VM with more space. The fact is: how much is enough?
Maybe placing the analysis in a place without sudo permission (no ‘root’), as described here?
I am stuck on this and I really don't know what else to do.
Thanks again for your kind help.
Best,
Rosie