A plesk subscription will have quota set in 2 places.

1. Subscription disk space quota

This is the total disk space allocated to a plesk subscription. This will usually be sufficiently high.

Value of disk space quota allocated to a subscription can be found as shown below.

2. Quota of the FTP user of the website

Usually quota of the FTP user will be set to a value smaller than quota of associated plesk subscription. We need to ensure that quota of FTP user is sufficiently high to ensure proper working of the website.

In the example show in this KB, we can see that FTP quota is set to a value lower than actual disk space usage.

We need to increase "Hard quota on disk space" of FTP account to sufficiently high value to ensure that website & FTP account are working fine