If you are exporting them from an earlier version, then it is not an error. Transferring Attributes between different main versions (e.g. between AC20 and AC21) is not supported. If you want to do this, first open the old project in the same version, and save it. After this they are both the same version, so you should be able to transfer attributes.
If it is not that, then it can be any number of things. If you send me the exported files, and the 2 plan files (the one you exported from and the one you want to copy it to), I can take a look.
Also, in the meantime, you can also browse for a PLN/TPL in the Attribute manager, not attribute files only. This way you might be able to move your attributes.
Daniel Alexander Kovacs
Professional Services Consultant
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