The first thing to remember is that your microphone needs some software to process its signal. In your situation that's likely to be Skype, Zoom, Teams or something similar and you might need to go into the settings for that specific software to tell it to use your laptop's built-in microphone as its sound source (rather than, say, looking for a signal from a non-existent external microphone). So it's possible that your built in mike is already working properly but not being recognised by the software that you're using.
Having got that out of the way though, start by right-clicking on the volume icon at the bottom right of your screen and selecting Recording Devices. You should see a line showing the microphone. You're hoping to see
(a) a green tick alongside it ; and
(b) a volume bar that moves up and down as you speak.
If you've got both of those, it's probably a specific software issue, as above.
Otherwise click on 'Microphone' and select 'Properties'. With the 'General' tab selected, check that 'Device usage' shows 'Use this device'. With the 'Levels' tab selected, check that the two sliders are positioned towards the right.
If you're still getting nowhere, you probably need to reinstall the audio driver. Download this file:
https://global-download.acer.com/GDFiles/Driver/Audio/Audio_Realtek_6.0.1.7040_W81x64_A.zip?acerid=635171415292247332&Step1=NOTEBOOK&Step2=ASPIRE&Step3=ASPIRE%20V3-551&OS=ALL&LC=en&BC=ACER&SC=EMEA_27
Double-click on it to extract the set-up file.
Double-click on the set-up file to run it.
Restart your laptop.
Check that the microphone is enabled, as per my second paragraph above.