Convert Watt to Ampere-hour for battery
W is a measure of power, therefore it's likely that it's rated in Wh (Watt-hour, energy).

Since Ah represent a charge (current * time) you can multiply it by the voltage to get Wh; conversely, divide the Wh by the voltage to get the Ah rating.

Looking at the datasheet, there seem to be a reason why a single Ah value is not specified: it varies depending on the load current (see table in the bottom).

The 21 W indicates that it can provide 21 W for 15 minutes (21/4=5.25 Wh) when the cell voltage is at least 1.67 V, times 6 cells means 10 V. This voltage is the minimum tolerated value, and influences the depth of the discharge and, in the long term, the lifetime of the battery.

If you look at the table, the battery is approximately 4 Ah.