The error message tells you in detail in what order the elements are supposed to be placed and how many of them are allowed. In other words, the ordering or amount of the elements inside the <web-app> of your web.xml is incorrect. For example, as per the error message, <servlet> needs to go before <servlet-mapping>. The ? suffix means that there may be zero or one of them. The * suffix means that there may be zero or many of them.
So, the example below is invalid:
<servlet>...</servlet>
<servlet-mapping>...</servlet-mapping>
<servlet>...</servlet>
<servlet-mapping>...</servlet-mapping>
<servlet>...</servlet>
<servlet-mapping>...</servlet-mapping>
While the example below is valid:
<servlet>...</servlet>
<servlet>...</servlet>
<servlet>...</servlet>
<servlet-mapping>...</servlet-mapping>
<servlet-mapping>...</servlet-mapping>
<servlet-mapping>...</servlet-mapping>