First thing is check that the device credentials are correct:
What version of AirWave are you using? If you're on 8.2.6, you can try doing SNMP walks from the AMPCLI to test the credentials in the database. To do this, you'll need to know the device's ID which is gotten from the URL when you view the device's monitor page.
Example URL:
https://airwave.lab.com/ap_monitoring?id=2421
The ID is 2421
In CLI: go to 'Enter Commands' (opt 11)
$ ?sw
sw <ap_id> args [-o fname ] ... - snmp walk protocol v1 - see 'man snmpbulkwalk'
sw2 <ap_id> args [-o fname ] ... - snmp walk protocol v2c - see 'man snmpwalk'
sw3 <ap_id> args [-o fname ] ... - snmp walk protocol v3 - see 'man snmpbulkwalk'
With '-o fname' the output will be saved in the CLI user directory.
$ sw2 2421 sysName
SNMPv2-MIB::sysName.0 = STRING: jamaica-test-aos65
If you get a valid output like above, then the credentials are correct. If you're on older code - it might just be easier to go to the device's management page and updating the creds with the correct creds.
If the above didn't resolve your issue, it could also be the version of SNMP you're using. Are you using the default SNMPv2c? If not, then you can double check what version AMP is using to communicate from the Group -> select group -> Basic tab. There is a subsection for each device brand and sometimes another breakout by device type with an SNMP dropdown that lets you choose v1, v2c, v3.