Set Dates on Spreadsheet Rows
General review
Generally the code is OK and problems are due to lack of experience rather than bad style or habit.
I am unfamiliar with SpreadsheetApp so you should wait in case someone that is adds another answer before you accept this answer, if so inclined.
Rather than submit this answer I took a quick look at the DOC for SpreadsheetApp (I hope I got the correct one) and added to this answer at bottom under heading Performance.
Review points

Take care with indentation. all code blocks should be indented (If indentation mess is due to snippets poor formatting ignore this line)

When using var declare them at the top of the function.

Use constants const for variables that do not change.

Use i, j only for indexes or counters, if you are accessing coordinates, row, column, or x, y use names to match the axis name.
In this case it looks like you are only indexing by row index. In the rewrite I use the variable name row. Then a counter called i to count 25 and allow for the skipped row every 26 rows.
BTW When using i, j in loops the convention is i for the inner loop and j is the outer.

Don't calculate the same value many times. Eg you calculate i + j 3 times.

Avoid single use variables.

Use while loops in preference to for loops especially if you are incrementing the counter manually in the loops block.
While loops are generally syntactically cleaner and are less complex under the hood (if you use let to declare the counter).
Note that when using while loops it is easy to forget to add the incrementation so take care to not forget to increment / decrement

Use arithmetic assignment operators in this case += (Addition assignment) i += 25 rather than i = i + 25;

The is no need to create a new Date object every time you make a change.


Date object
The Date objects call Date.getFullYear always returns a Number as an integer.
There is no need to use parseFloat and the result of !isNaN(parseFloat(yearCompleted)) will always be true and is thus not required.
Further the variable yearCompleted is only used in the unneeded predicate and as such is not required at all.
To set the date to the first day of the month you don't need to use year, month + 1, 0 just set it to the 1st year, month, 1
The inner if statement
It is unclear if the call to get interval = ssData.getRange(i + j, 5).getValue() will return a number or not.
Because the use of monthCompleted is dependent on the valid numerosity of interval you can combine the two in one expression. However I suspect that interval will always be a number and as such there is no need for the isNaN test.
Rewrite
Using the above points the rewrite greatly reduces the code complexity. Without data there is no way to know if there is a need to vet the interval value. I added the test but it may not be needed.
function setDates() {
    var row = 4, i;
    const data = SpreadsheetApp.getActiveSpreadsheet().getSheetByName(&quot;Data&quot;);
    while (row &lt; data.getLastRow()) {
        i = 25;
        while (i--) {
            const interval = data.getRange(row, 5).getValue();
            if (!isNaN(interval)) {
                const completed = new Date(data.getRange(row, 4).getValue()); 
                completed.setMonth(interval + completed.getMonth());
                completed.setDate(1);
                data.getRange(row, 6).setValue(completed); 
            }
            row ++;
        }
        row ++;
    }
}

Performance
I have taken a quick look at the SpreadsheetApp docs and thus will address the performance problems you are concerned about.
The problem I think is the use of getRange(row, column) which you use to get each cell.
Rather...

Use getRange(row, column, numRows, numColumns) to get all the rows and columns in one call before you iterate the cells.

Then get an array of values from the range using range.getValues() make the modifications needed.

Then use range.setvalues() to set the changes made


This may get a significant performance increase.
Rewrite based on Docs
As I have no data or access to the API the rewrite is only a guess based on a quick read of the DOC's
The only problem I can foresee is why you skip every 26th row and why you start from the 5th row (index as row 4)
function setDates() {
    var rowIdx = 0;
    const sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName(&quot;Data&quot;);
    const range = sheet.getRange(4, 4, sheet.getLastRow() - 4, 3);
    const data = range.getValues(); // as 2D array of rows by columns eg...
                                    // data[0][0] is row 4 column 4
                                    // data[10][2] is row 14 column 6
    for (const row of data) {
        if (rowIdx &gt; 0 &amp;&amp; (rowIdx % 26)) { // skip every 26th row
            const interval = row[1];
            if (!isNaN(interval)) {
                row[2] = new Date(row[0]);
                row[2].setMonth(interval + row[2].getMonth());
                row[2].setDate(1);
            }
        }
        rowIdx++;
    }
    range.setValues(data);
}