If your Shopify store and Facebook/Google ad account(s) use different currency, you can handle by setting an exchange rate.
Visit Cost settings > Facebook or Cost settings > Google Ads.
In Integration Exchange Rate, put in the exchange rate between your store's currency and ad account's currency.
Eg. If your store is in USD and Facebook ad account is in EUR, then:
1 USD equals 0.91 EUR (as at 4 May 2020)
1 Shopify currency unit equals 0.91 Facebook currency units
If you use the same currency across all platforms, leave the exchange rate at 1.
Check out the list of exchange rates at General settings > Currency.