You will get this error message if you have reached the limit on how many calls Weighsoft (as well as any other system linked to Xero) can make through the Xero API.
Xero is designed for smaller businesses and as such has several limitations on how many API calls can be made per minute/day and how many invoices can be posted per month:
According to the Xero website the limitations are as follows:
1000 Invoices per month
2000 Bank Transactions per month
Concurrent Limit: 5 calls in progress at one time
Minute Limit: 60 calls per minute
Daily Limit: 1000 calls per day (this can be increased with other Xero packages)
In terms of the API a "Call" is when any datapoint is transferred between Xero and Weighsoft in either direction. So synching a single account will contain several calls due to each point of data (account number, account name, balance etc) being treated separately.
If any of the above limits are hit then the API will not allow any further traffic between Weighsoft. For example, if you were to hit the 1000 calls per day limit due to account syncs, then you would not be able not post any invoices.