Present i am having test complete desk top license , looking for web.my question is after buying web

In general, you should be able to buy a Web license and add it to the other licenses on your License Manager, or to the node-locked licenses on your local machine.

To perform Web testing, you would need to have installed the Web module on your machine.  Check "File | Install Extensions" to make sure the Web extensions are present.  If not, modify your TestComplete installation to install the Web module.