[SalesForce] How to use mobile connect SMS in Journey Builder

This is as per design - only outbound templates are supported. Refer the official documentation where it states

How to Configure MobileConnect for Use with Journey Builder

Follow the steps below to properly configure your MobileConnect messages for use with Journey Builder:

Contact your Salesforce Marketing Cloud relationship manager to ensure your account allows access to both MobileConnect (using the upgraded version of Contacts) and Journey Builder.

Ensure the MobileConnect Demographic data extension contains the contact data used to personalize any messages used as part of Journey Builder.

Ensure that all contacts you wish to participate in your Journey Builder activities subscribe to the appropriate short or long code and keyword.

You can specify those subscriptions at import or by using messages such as the Mobile Opt-In template.

Create all Outbound MobileConnect messages using the Interaction send method to permit Journey Builder access.

Note that all contacts must initially subscribe to your short or long code and keyword to receive messages at the beginning of the journey. The contacts must also remain subscribed continuously to your short or long code and keyword to continue receving messages as part of the Journey Builder activity. If the contact unsubscribes at any time, that contact will not receive additional messages.

Once you properly configure your MobileConnect messages, those messsages become available for addition to Journey Builder automatically:

Use the Send SMS activity within Journey Builder. Click Configure to select the message you wish to send within that activity. Journey Builder will display a list of all messages using the Interaction send method.

I'd suggest trying the MOBILE_NUMBER personalization string for your lookup value.

%%[ 
    SET @idSubscriber = MOBILE_NUMBER 
    SET @rows = LookupOrderedRows("DE", 1, "ID_SUBSCRIBER Desc", "ID_SUBSCRIBER", @idSubscriber) 
    SET @RowCount = RowCount(@rows) 
    IF (Rowcount(@rows) > 0) THEN 
        VAR @ILRow, @name, @limit 
        SET @ILRow = row(@rows,1) 
        SET @name = Field(@ILRow, "DES_NOMPILA") 
        SET @limit = Field(@ILRow, "LIM_MC") 
    ENDIF 
]%%
1= %%=v(@idSubscriber)=%% 2= %%=v(@RowCount)=%%

The MOBILE_NUMBER is the subscriber key for SMS subscribers, since SMS and email opt-ins and opt-outs are handled separately in SFMC.