There are some limitations when using Customer Attributes.
When you upgrade to Analytics Premium, there is a 24-hour delay before additional attributes are available. You may see an Attribute Subscription Max error issued during this delay.
For more information you may have look into Frequently asked questions, limitations, and best practices
hope this works for you.
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When configuring subscriptions and selecting attributes, the following limits apply, depending on the solutions you own:
Standard: 3 total
Premium: 200 per report suite
Target Standard: 5
Target Premium: 200
I think, after upgrading to analytics premium you just need to wait for availability of new additional attributes, in meantime this error may occur.
Guess the problem is related to Schema Validation.
Try to upload the file again and see if you are able to solve the problem since it is Time Out error. Else check whether everything is fine for the field highlighted above in your screen grab.
* Each attribute source should contain the same number of comma-separated fields.
* Fields containing a line-break, double-quote, or commas must be quoted.
* Double-quote characters in a field must be escaped using a backslash (\).