The depreciation rate/year has been left blank under Accounting Policies, hence the warning message. 

You may update the Depreciation rates under Settings>>Report Settings>>Accounting Policies>>Balance Sheet Accounting Policies>>Tangible fixed assets by selecting the green button (update as zero in the case there is no depreciation) and save it. 

Once done, please regenerate the accounts report to see the impact.

Click here to watch how to change the method of depreciation